[Claude Code - Stage 3] Parity hardening (cassettes + edge cases)

[ashwing]

Part of #113 (Claude Code integration).

Extend the Messages cassette matrix beyond Stage 2's single acceptance recording (same recorder approach as the Responses cassettes in #94) to the edge cases:

• Interleaved thinking + tool_use in one turn.
• Multi-round gateway tools (model calls a gateway tool on consecutive turns).
• Streaming tool_use (input_json_delta partial-JSON args).
• system prompt handling (attribution block + multi-block system).

Follows #115 (which lands the recorder + the first gateway-tool cassette).