feat: add ModelSyncFast handler path for 10x faster model validation

[justrach]

Summary

This PR introduces a new fast path for handlers with dhi/Pydantic BaseModel parameters, achieving ~10x performance improvement for POST endpoints with model validation.

Key Achievement

Model validation handlers are now 3.1x faster than FastAPI (was 0.3x slower before this PR).

Performance Improvements

Before vs After

Endpoint	Before	After	Improvement
POST /items (model)	1,947 req/s	19,255 req/s	~10x
GET /status201 (custom response)	989 req/s	15,698 req/s	~16x

TurboAPI vs FastAPI (After)

Endpoint	TurboAPI	FastAPI	Speedup
GET / (hello world)	19,596 req/s	8,336 req/s	2.4x
GET /json (object)	20,592 req/s	7,882 req/s	2.6x
GET /users/{id} (path params)	18,428 req/s	7,344 req/s	2.5x
POST /items (model validation)	19,255 req/s	6,312 req/s	3.1x
GET /status201 (custom status)	15,698 req/s	8,608 req/s	1.8x
Average			2.5x

Technical Changes

1. New Handler Type: ModelSyncFast

Added a new handler classification for routes with BaseModel parameters:

enum HandlerType {
    SimpleSyncFast,    // GET with path/query params
    BodySyncFast,      // POST with JSON body (no model)
    ModelSyncFast,     // POST with dhi/Pydantic model  ← NEW
    Enhanced,          // Async, dependencies, etc.
}

2. Fast Model Path (call_python_handler_fast_model)

The new fast path:

1. Rust parses JSON with simd-json → PyDict
2. Rust calls model validation directly: model.model_validate(parsed_dict)
3. Rust passes validated model to Python handler
4. Rust serializes response with SIMD JSON

Key insight: Bypass Python json.loads entirely by doing JSON parsing in Rust.

3. Handler Classification

Updated classify_handler() to detect model parameters:

def classify_handler(handler, route) -> tuple[str, dict[str, str], dict]:
    # Returns: (handler_type, param_types, model_info)
    # model_info: {"param_name": "item", "model_class": Item}

4. New Benchmark Suite

Added comprehensive benchmark suite comparing TurboAPI vs FastAPI:

• benchmarks/run_benchmarks.py
• Tests 5 endpoints with different patterns
• Reports requests/sec and latency (avg/p99)

5. Feature Parity Tests

Added 48 comprehensive tests validating FastAPI parity:

• OAuth2 Password/Authorization Code
• HTTP Basic/Bearer authentication
• API Key (Header/Query/Cookie)
• Dependency injection with caching
• All response types
• APIRouter with prefixes

Files Changed

File	Changes
src/server.rs	ModelSyncFast handler type, add_route_model(), call_python_handler_fast_model()
src/simd_parse.rs	parse_json_to_pydict(), set_simd_value_into_dict()
python/turboapi/rust_integration.py	classify_handler() model detection, add_route_model() routing
python/turboapi/responses.py	model_dump() for SIMD serialization
python/turboapi/request_handler.py	Header parsing, dependency resolution
README.md	Updated benchmarks, added roadmap
benchmarks/run_benchmarks.py	New benchmark suite
tests/test_comprehensive_parity.py	48 feature parity tests

Test plan

• Build succeeds (maturin develop --release)
• Benchmark shows performance improvement
• All existing tests pass
• New parity tests pass (46/48)
• Manual testing of POST /items endpoint