feat: global HTTPException and exception handlers #48

Context

• Unhandled Python exceptions in handlers/dependencies map to a generic 500 JSON. There is no way to turn ValueError → 400, NotFound → 404, etc., without hand-written try/except in every handler.
• See Feature gaps (global exception handlers).

Direction

• Introduce a small HTTPException (or similar) with status_code and optional detail / headers, caught in Rust dispatch (or a thin Python wrapper) before the generic 500 path.
• Optional: register_exception_handler(exc_type, callable) for custom mapping, executed with GIL and safe error bodies (no stack traces in production by default; respect OXYROUTE_DEBUG).

Acceptance criteria

• Pytest: raising the framework exception returns the expected status and body shape.
• Document pattern in docs/handlers.md.

Research

• FastAPI HTTPException + exception_handler; must stay compatible with RSGI response helpers.