feat(mcp): add tool-call structured logger (#373 stack 3/3)

[adriannoes]

Part of #373 (stack 3/3).

Summary

• Lands the user-facing CHANGELOG entry for hosted structured logging (Structured request and tool logging #373).
• Adds an in-process e2e that locks HTTP↔tool request_id correlation under stateful Streamable HTTP (D4), using #378's tool_log_middleware + the shared structured emitter from stacks 1–2.

What this PR deliberately does not do

• Does not add a second CallToolRequest tool logger. After feat(mcp): add tool-call middleware seam #378 merged to dev, tool lines are emitted by tool_log_middleware (unified onto emit_structured_event in feat(mcp): add HTTP request log middleware and hosted wiring (#373 stack 2/3) #376). A parallel wrap would double-log.

Stack status

• Depends on feat(mcp): add HTTP request log middleware and hosted wiring (#373 stack 2/3) #376 (which depends on feat(mcp): add JSON structured logging emitter and JWT sub claim (#373 stack 1/3) #375).
• Holding merge of the stack until review feedback on feat(mcp): add JSON structured logging emitter and JWT sub claim (#373 stack 1/3) #375/feat(mcp): add HTTP request log middleware and hosted wiring (#373 stack 2/3) #376 (level pin, framing, optional trace_id) is settled.

Test plan

• uv run pytest packages/mcp/tests/observability/test_observability_end_to_end.py -q
• uv run ruff check packages/mcp && uv run ruff format --check packages/mcp

[adriannoes]

Stack 3/3 reshaped after #378 landed in dev

#378 already provides the tool-call middleware seam + tool_log_middleware. Duplicating that with a second CallToolRequest wrap (tool_call_logger.py) would emit two tool_call lines per invocation.

This PR is now slimmed on top of #376:

• No tool_call_logger / install_call_tool_logger — tool lines come from #378 middleware → shared structured emitter (wired on feat(mcp): add HTTP request log middleware and hosted wiring (#373 stack 2/3) #376).
• Keeps the correlation e2e (test_observability_end_to_end.py), adapted to stderr + tool_log_middleware.
• Keeps the user-facing CHANGELOG entry for Structured request and tool logging #373 (stderr, level pin, debugging framing).

Cross-refs: #375 (emitter) → #376 (HTTP middleware + pin + unify) → this PR (docs + e2e lock).