feat: expose tool_call_id on RunContextWrapper for lifecycle hooks

[nileshpatil6]

Summary

Closes #1849.

on_tool_start and on_tool_end hooks receive a RunContextWrapper as their context parameter. When the invocation is a function-tool call, the runtime actually passes a ToolContext (a subclass) which already carries tool_call_id. However, users who want to correlate parallel tool calls in observability/metrics code were forced to write:

async def on_tool_start(self, context, agent, tool):
    if isinstance(context, ToolContext):          # painful isinstance guard
        call_id = context.tool_call_id

This PR adds tool_call_id: str | None as an init=False field (default None) directly on RunContextWrapper, so the pattern becomes:

async def on_tool_start(self, context, agent, tool):
    call_id = context.tool_call_id  # str for function tools, None otherwise

Changes

• src/agents/run_context.py — adds tool_call_id: str | None = field(default=None, init=False) to RunContextWrapper; propagates the value through _fork_with_tool_input and _fork_without_tool_input.
• tests/test_run_hooks.py — adds two tests:
  • test_tool_call_id_exposed_on_run_context_wrapper — verifies the correct call_id is surfaced in both on_tool_start and on_tool_end without any isinstance check.
  • test_tool_call_id_is_none_outside_tool_context — verifies the field is None on a plain RunContextWrapper.

Compatibility

• ToolContext redeclares tool_call_id as a required init=True field (str, not str | None), so subclass behaviour and construction are unchanged.
• The new base field uses init=False to avoid clashing with ToolContext.from_agent_context, which copies base-class init fields by name.
• All existing tests pass. The two sandbox/Unix-only test files (tests/sandbox/, tests/test_run_state.py, tests/test_sandbox_memory.py) fail due to the missing fcntl module on Windows — this is pre-existing and unrelated to this change.

Test plan

• uv run pytest tests/test_run_hooks.py tests/test_function_tool.py tests/test_tool_context.py — 68 passed
• uv run pyright src/agents/run_context.py src/agents/tool_context.py — 0 errors

[seratch]

We're aware of this pain point, but we still don't want to add the property to the parent level. We will consider changing the type hints when releasing a major version with migration guide.