kimi-k2.7-code generates tool_use IDs that block subsequent requests to Anthropic with HTTP 400

### Issue Description
When using `kimi-k2.7-code` during an agentic Copilot session, tool calls generate IDs containing dots and colons (e.g. `functions.list_dir:0`).

When switching models or replaying conversation history to an Anthropic endpoint (like Claude 4.6 Sonnet), the API rejects the request with HTTP 400:
`invalid_request_error: messages.X.content.Y.tool_use.id: String should match pattern '^[a-zA-Z0-9_-]+$'`

Because VS Code preserves the conversation history with the un-sanitized Kimi IDs, all subsequent requests in that session fail permanently until a new chat session is started.

### Expected Behavior
VS Code / Copilot extension should sanitize outgoing `tool_use.id` strings in message history (e.g., replacing invalid characters with `_` or re-mapping IDs) to ensure compliance with Anthropic's `^[a-zA-Z0-9_-]+$` regex constraint before sending history payloads.

- Copilot Chat Extension Version: 0.55.0
- VS Code Version: 1.130.0
- OS Version: macOS 26.5.2
- Feature: Agent mode (Code agent)
- Selected model: kimi-k2.7-code (model that produces the invalid IDs); error surfaces when any subsequent request is made in the same session
- Logs:
```
Sorry, your request failed. Please try again.

Client Request Id: 3a35965a-8ca7-4f10-9fe3-9e856b98fe19

GH Request Id: FAE9:BB6DC:808FD0:8F4CBC:6A6718AA

Reason: Request Failed: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.69.content.0.tool_use.id: String should match pattern '^[a-zA-Z0-9_-]+$'"},"request_id":"req_vrtx_011CdSFwojHENqbKTfaoaucd"}
```

Debug log evidence from session {SESSION_UUID}/main.jsonl:

`kimi-k2.7-code` generated the following `tool_use` IDs in agent responses:
```
functions.list_dir:0
functions.list_dir:1
functions.grep_search:2
functions.read_file:3
functions.create_file:15
```
These IDs contain `.` (dot) and `:` (colon), which violate the pattern required by Anthropic `^[a-zA-Z0-9_-]+$`.

---

**Steps to Reproduce:**

1. Open a VS Code Copilot Chat panel in **Agent / Code mode**.
2. Select **`kimi-k2.7-code`** as the model.
3. Send a prompt that causes the agent to make multiple tool calls (e.g. any coding task that reads files or runs searches). Observe that kimi completes the turns successfully - the IDs are accepted by kimi's own endpoint.
4. Switch to an Anthropic model, eg. `claude-sonnet-4.6`
5. Send a **follow-up user message** in the same session (any message).
6. Observe: the request fails immediately with HTTP 400 — no `llm_request` is logged; VS Code shows _"Sorry, your request failed. Please try again."_
7. Clicking **"Try Again"** fails identically as long as the session history is unchanged.
---

**Root cause (from log analysis):**

`kimi-k2.7-code` uses the ID format `functions.:` (e.g. `functions.list_dir:0`). When VS Code replays this conversation history to the GitHub Copilot API endpoint for any subsequent request, the Anthropic API validates all `tool_use.id` values in the history and rejects the payload.

**Expected behaviour:** The Copilot extension should sanitize `tool_use.id` values when replaying cross-model conversation history, replacing any characters outside `[a-zA-Z0-9_-]` with `_`. This is a one-liner fix on the client side.

**Workaround:** Start a new chat session after the error occurs; "Try Again" alone is not sufficient unless VS Code switches models automatically.