Bug: tools declared by a custom_agents agent are announced but not callable by the model

# Bug: tools declared by a `custom_agents` agent are announced but not callable by the model

## Summary

When a session is created with `custom_agents=[...]` plus `agent=""`, the tools that agent
declares in its own `tools:` list are **not usable by the model**. The runtime reports them as
selected (a `subagent.selected` event lists them verbatim), but the model behaves as though it has
no tools at all: it makes **zero tool calls** and states plainly that it has no tool capable of
doing the work.

Passing the **identical** tool list via `available_tools=[...]` on the same model, same prompt,
same sandbox works correctly.

The failure is silent. There is no error, no warning, and no rejected tool call — the model simply
answers in prose. A caller that only inspects the final message (or a `subagent.selected` event)
sees a perfectly healthy-looking session.

## Environment

| | |
|---|---|
| Python SDK | `github-copilot-sdk` 1.0.11 |
| CLI / server | `copilot` 1.0.79 (also reproduced on 1.0.78) |
| Connection | `RuntimeConnection.for_uri(...)` to a `copilot --headless --server` process in a container |
| Models | reproduced with `gpt-5.3-codex` **and** `gpt-5.4` |
| `agent_mode` | `"autopilot"` |
| Permissions | `PermissionHandler.approve_all` |
| OS | server on Linux (Ubuntu 24.04 container); client on Windows |

## Reproduction

`repro.py` below runs the same request twice against the same server: once with `custom_agents`,
once with `available_tools`. Start a headless server first, then run it.

```bash
# a copilot --headless --server on :3000, published to 18097, with a /workspace/repo cwd
docker run -d --rm --name ghcprepro -p 18097:3000 \
-e COPILOT_SDK_AUTH_TOKEN="$GITHUB_TOKEN" \
-e COPILOT_CONNECTION_TOKEN="repro123" \

python repro.py gpt-5.3-codex
docker exec ghcprepro sh -c 'ls /workspace/repo/' # which file actually got created?
```

```python
"""Minimal A/B: custom_agents' declared tools vs the identical list via available_tools."""
import asyncio, sys
from copilot import CopilotClient, RuntimeConnection
from copilot.session import PermissionHandler
from copilot.session_events import SessionEventType

URI, TOKEN = "localhost:18097", "repro123"
MODEL = sys.argv[1] if len(sys.argv) > 1 else "gpt-5.3-codex"
TOOLS = ["builtin:view", "builtin:grep", "builtin:glob",
"builtin:edit", "builtin:create", "builtin:apply_patch", "builtin:skill"]

WRITER_AGENT = {
"name": "writer",
"description": "Creates files on disk.",
"tools": TOOLS, # <-- identical list to available_tools in case B
"model": MODEL,
"prompt": "You are the Writer Agent. You create files on disk with your file tools.",
}

ASK = ("Create a file named {name} in the current directory containing exactly the text 'hello'. "
"Then tell me in one sentence which tool you used, or state plainly that you have no tool "
"capable of creating a file.")

async def run_case(name: str, *, use_custom_agent: bool) -> str:
connection = RuntimeConnection.for_uri(URI, connection_token=TOKEN)
async with CopilotClient(connection=connection, log_level="error") as client:
kwargs = {"on_permission_request": PermissionHandler.approve_all, "model": MODEL,
"streaming": True, "working_directory": "/workspace/repo"}
if use_custom_agent:
kwargs["custom_agents"] = [WRITER_AGENT]
kwargs["agent"] = "writer"
else:
kwargs["available_tools"] = TOOLS
session = await client.create_session(**kwargs)

done, text = asyncio.Event(), []
def on_event(e):
if e.type == SessionEventType.ASSISTANT_MESSAGE: text.append(e.data.content or "")
elif e.type in (SessionEventType.SESSION_IDLE, SessionEventType.SESSION_ERROR): done.set()
session.on(on_event)
await session.send(ASK.format(name=name), agent_mode="autopilot")
await asyncio.wait_for(done.wait(), timeout=120)
return f"session={session.session_id} :: {' '.join(text).strip()[:200]}"

async def main():
print("A custom_agents ->", await run_case("case_a.txt", use_custom_agent=True))
print("B available_tools->", await run_case("case_b.txt", use_custom_agent=False))

asyncio.run(main())
```

## Actual result

```
A custom_agents -> session=41113715-... :: I have no tool capable of creating a file.
B available_tools-> session=396f85a7-... :: Creating `case_b.txt` now ... I used the `apply_patch`
tool to create `case_b.txt` with exactly `hello`.

$ ls /workspace/repo/
case_b.txt # case_a.txt was never created
```

Case A reproduces on every attempt (3/3 trials, plus both models above).

### The contradiction, from the server's own session log

`~/.copilot/session-state//events.jsonl` for the **failing** case A session announces
exactly the tools that the model then cannot use:

```json
{"type":"subagent.selected","data":{"agentName":"writer","agentDisplayName":"writer",
"tools":["builtin:view","builtin:grep","builtin:glob","builtin:edit","builtin:create",
"builtin:apply_patch","builtin:skill"]}}
```

Tool invocations recorded in each session:

| session | `tool.execution_start` entries |
|---|---|
| A (custom_agents) | **none at all** |
| B (available_tools) | `apply_patch` ×1 |

So the agent's tool set is resolved and reported, but never reaches the model's callable tool set.

## Expected result

An agent's declared `tools:` should be callable by the model in that agent's turn — equivalent to
passing the same list via `available_tools`. Case A should create `case_a.txt`.

Failing that, a session whose agent declares tools the runtime cannot expose should raise an error
at `create_session`, rather than silently producing a tool-less agent.

## Impact

This is expensive to diagnose because every observable signal says the session is healthy: the
agent is selected, the declared tools are echoed back, `agent_mode` is `autopilot`, permissions are
auto-approved, and the model returns a confident, well-formed answer. Only the filesystem (or the
absence of `tool.execution_start` events) reveals that nothing happened.

In our pipeline this manifested as agents that "completed" substantial work in ~18 seconds while
writing nothing to disk, and as automated fix-up steps that never repaired anything. Because each
affected stage then failed a downstream check for an unrelated-looking reason, it took a long time
to trace back to tool availability.

## Workaround

Do not use `custom_agents` for any agent that needs tools. Pass the tool list via
`available_tools` on `create_session`, and supply the agent's instructions as a normal system
message instead of an agent definition.

## Secondary observation (possibly intended, but surprising)

`builtin:create` alone is not sufficient for file creation — with
`[view, grep, glob, edit, create]` the model reports it has no way to create a file, and creation
only succeeds once `builtin:apply_patch` is included (the model then uses `apply_patch`). If
`create` is not independently usable, it would help for its absence/aliasing to be documented, or
for `create` to be rejected as unknown rather than accepted silently.

Bắt đầu với repro.py và chạy các trường hợp A/B của create_session bằng custom_agents so với available_tools. Theo dõi cách các tùy chọn đó đi qua create_session, sau đó so sánh các công cụ của agent được chọn với các công cụ có thể gọi trong session-state//events.jsonl. Được xem là hoàn thành khi các công cụ đã khai báo có thể được gọi trong lượt của custom agent, hoặc create_session từ chối cấu hình không được hỗ trợ.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.