US-011: Performance optimization for API request serialization

Added criterion benchmarks and optimized flatten_tool_result_content:
- Added criterion dev-dependency and request_building benchmark suite
- Optimized flatten_tool_result_content to pre-allocate capacity and avoid
  intermediate Vec construction (was collecting to Vec then joining)
- Made key functions public for benchmarking: translate_message,
  build_chat_completion_request, flatten_tool_result_content,
  is_reasoning_model, model_rejects_is_error_field

Benchmark results:
- flatten_tool_result_content/single_text: ~17ns
- translate_message/text_only: ~200ns
- build_chat_completion_request/10 messages: ~16.4µs
- is_reasoning_model detection: ~26-42ns

All 119 unit tests and 29 integration tests pass.
cargo clippy passes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

rust/crates/api/Cargo.toml

@@ -13,5 +13,12 @@ serde_json.workspace = true
 telemetry = { path = "../telemetry" }
 tokio = { version = "1", features = ["io-util", "macros", "net", "rt-multi-thread", "time"] }
 
+[dev-dependencies]
+criterion = { version = "0.5", features = ["html_reports"] }
+
 [lints]
 workspace = true
+
+[[bench]]
+name = "request_building"
+harness = false