test: add Anthropic Messages recording mode + gateway-tool cassettes

[ashwing]

Summary

Part of #115 (Claude Code — Stage 2). Adds a messages mode to the cassette recorder for the Anthropic Messages API (/v1/messages), and records the upstream gateway-tool traffic that the Claude Code server-side tool loop will replay in tests.

The Stage 2 server-side loop itself is gated on making the executor loop API-agnostic (it's currently typed to Responses RequestPayload/ResponsePayload) — that's a core refactor for a separate change. Recording the acceptance cassette has no such dependency and is the unblocked prerequisite #115 calls out, so this lands the recorder + fixtures ahead of the loop.

What's here:

• messages recorder mode (record_cassette.py): sends Anthropic /v1/messages with tools, stateless (resends full messages history each turn). With --tool-outputs, a turn following a tool_use feeds back matching tool_result blocks keyed by tool name — mirroring how a gateway tool loop feeds results back. Streaming reconstructs the final message from the Anthropic SSE (content_block_delta / input_json_delta / message_delta).
• Recorded cassettes (messages/): a two-turn web_search gateway-tool session — model emits the gateway tool_use, tool_result is fed back, model produces the final answer. Both streaming and non-streaming.
• Reproducer record_messages_cassettes.sh + a README modes-table row.

Test Plan

• Recorded against vLLM 0.25.1 (the current release, which serves /v1/messages natively), Qwen/Qwen3-30B-A3B-FP8, hermes tool parser + qwen3 reasoning parser.
• python -m py_compile record_cassette.py clean; --help shows the new mode; cargo build --workspace unaffected (no Rust changed).
• Recorded cassettes verified: 2 turns each, POST /v1/messages, turn 1 emits tool_use(web_search) with stop_reason: tool_use, turn 2 request carries the full Anthropic history (user → assistant with tool_use → user with tool_result), turn 2 produces the final end_turn answer.
• No Rust consumer yet: the Messages cassette loader lands with the Stage 2 loop these fixtures are for. Recording them now (per [Claude Code - Stage 2] Server-side gateway tool loop for Messages #115's carved-out sub-task) de-risks that work and keeps the fixtures reproducible.
• Note: streaming reasoning arrives as <think>-tagged text deltas while non-streaming arrives as thinking blocks — this is an accurate capture of what vLLM emits on each path, recorded intentionally so the future parser handles both.