AgentTool: option to propagate parent conversation history to child session

## 🔴 Required Information

### Is your feature request related to a specific problem?

The problem is the following: `AgentTool` starts the child agent with a blank session. The child only sees `args["request"]`, which is a string generated by the parent LLM and, therefore, that is rephrased. This means the following:

- The child has no idea what the user actually said in prior turns.
- Emphasis and constraints can get lost in the parent's rephrasing (For example, imagine the user said something like *"MUST have 2 bedrooms, won't compromise"* can become *"find apartment with 2 bedrooms"*).
- This also impact LLM-oriented Confirmation flows and multi-turn extraction tasks, because the child can't reference earlier messages.

Today the only workaround is subclassing `AgentTool` and manually injecting parent events into the child session — multiple people have done this independently (see #3878, where a contributor built a `ContextAwareAgentTool` and noted *"For a long-term solution, we need a new pattern to be established in ADK"*).

**PS:** Although `sub_agents` share conversation context, they have fundamentally different dynamics — a sub-agent takes over the conversation via `transfer_to_agent` and the parent is out of the loop until control returns. `AgentTool` is the opposite: the parent calls the child, gets a result, and continues its own reasoning. It's a tool call, not a handoff. These are two distinct interaction patterns, and the tool-call pattern would benefit from optionally having conversation history available without changing its control-flow semantics.

### Describe the Solution You'd Like

A new opt-in boolean on `AgentTool`:

```python
AgentTool(agent=my_agent, propagate_conversation_history=True)
```

When `True`, user/model text events from the parent session are copied into the child's ephemeral session before execution. Same pattern as the existing `propagate_grounding_metadata` and `include_plugins` flags.

Key constraints:
- Defaults to `False` — existing behavior unchanged.
- Excludes `function_call`/`function_response` events (they'd reference tools the child doesn't have).
- Creates fresh `Event` objects (no `state_delta` leakage).
- Filters out `thought` parts.

### Impact on your work

We have production agents where a parent routes to specialized tool agents for extraction tasks. Without conversation context, the tool agents consistently misclassify user intent. The subclass workaround works but breaks on ADK upgrades. A first-class parameter would let us remove fragile internal-API dependencies.

### Willingness to contribute

Yes — I have a working implementation with tests and will open a PR right away.

---

## 🟡 Recommended Information

### Describe Alternatives You've Considered

| Alternative | Why it doesn't work |
|---|---|
| `sub_agents` (handoff) | Transfers full control to the child — parent can never route again. Incompatible with `AgentTool`'s call-and-return contract. |
| `mode='chat'` | Has unresolved scheduler bugs (#5780), only applies to `LlmAgent` callers, and changes semantics (multi-turn vs. single-turn with context). |
| Persistent sessions (PR #2849, closed) | Solves a different problem (cross-invocation caching). We only need current-call context injection — no lifecycle or cleanup concerns. |
| Subclass workaround | Brittle — duplicates framework internals, breaks on upstream refactors. |

### Proposed API / Implementation

The implementation is ~30 lines of new code, following the `propagate_grounding_metadata` pattern exactly:

1. New kwarg on `AgentTool.__init__` and `_TaskAgentTool.__init__`
2. A small `_inject_conversation_history` helper
3. A 3-line call site in `run_async` after `create_session`

No changes to `_SingleTurnAgentTool`, `AgentToolConfig`, or `from_config` (matching precedent).

### Additional Context

Related issues:
- #3878 — community workaround + request for first-class support
- #5780 — context loss confirmed as known v2 regression
- #3954 — requests persistent conversation for AgentTool
- #2130 — established "isolation by design" (we respect this via opt-in default)

Precedent:
- `d689a04f` — `propagate_grounding_metadata` addition
- `777dba30` — `include_plugins` addition