feat(errors): register exception handlers by exception type

[ZhuchkaTriplesix]

Context

• HTTPException maps to status + JSON body (oxyroute/exceptions.py, src/dispatch.rs).
• Other exceptions → 500 with safe body (OXYROUTE_DEBUG for detail).
• docs/feature.md notes missing register_exception_handler (partial scope of feat: global HTTPException and exception handlers #48).

Problem

Users cannot register handlers for domain-specific exception types (e.g. NotFoundError, ValidationError) without wrapping every route handler.

Proposed change

• App.register_exception_handler(exc_type, handler) storing a type → callable registry.
• On PyErr from route / dependency / middleware, walk the registry (subclass match) and map to Response / dict / str like normal handlers.
• Production 500 remains safe when no handler matches.

Files

• oxyroute/app.py, oxyroute/exceptions.py, src/dispatch.rs, src/lib.rs
• docs/handlers.md, tests

Acceptance criteria

• Custom exception maps to expected status and body.
• Subclass matching is predictable (most specific wins).
• Unhandled exceptions still return safe 500 JSON.
• Docs include a minimal example.

Framework impact

Centralized error handling for domain and validation errors.