mizan-fastapi: more Pythonic and declarative

Reworked the MVP code along the lines Ryth flagged. Same behavior
(11/11 tests still pass), tighter idiom.

executor.py:
- Replaced FunctionResult / FunctionError dataclasses with a MizanError
  exception hierarchy (NotFound, BadRequest, ValidationFailed,
  Unauthorized, Forbidden, NotImplementedYet, InternalError). Each
  carries its own ErrorCode + HTTP status; the dispatcher path raises
  rather than returning sentinel objects.
- Auth check uses match/case for the requirement (True / 'staff' /
  'superuser' / callable / other) — single declarative dispatch instead
  of an if/elif chain.
- Broke up the single 80-line execute_function into focused helpers:
  _resolve_function, _enforce_auth, _validate_input, _serialize,
  _invalidation_target. The execute_function body now reads as five
  declarative steps.
- Input validation uses Pydantic's model_fields[name].is_required()
  directly and a list comprehension for required-field reporting,
  instead of round-tripping through model_json_schema().

router.py:
- POST /call/ now declares its body as a Pydantic CallBody model;
  FastAPI handles parsing + envelope validation. No more manual
  await request.json() + dict[get] dancing.
- Endpoint bodies shrink to 3-5 lines each. Context fetch uses a
  dict comprehension over the function group.
- mizan_exception_handler renders MizanError to the protocol's
  {error: {code, message, details}} envelope.
- mizan_validation_handler maps FastAPI's RequestValidationError to
  the same envelope under BAD_REQUEST so the wire format is uniform
  whether the failure is body-shape or business validation.

__init__.py: exposes the full exception hierarchy + both handlers
so consumers can wire them onto their FastAPI app declaratively:

    app.add_exception_handler(MizanError, mizan_exception_handler)
    app.add_exception_handler(RequestValidationError, mizan_validation_handler)

Verified: mizan-core 15/15, mizan-django 348 pass, mizan-fastapi 11/11.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backends/mizan-fastapi/tests/test_dispatch.py

@@ -4,12 +4,18 @@ from __future__ import annotations
 
 import pytest
 from fastapi import FastAPI
+from fastapi.exceptions import RequestValidationError
 from fastapi.testclient import TestClient
 from pydantic import BaseModel
 
 from mizan_core.client.function import client
 from mizan_core.registry import clear_registry, register
-from mizan_fastapi import router as mizan_router
+from mizan_fastapi import (
+    MizanError,
+    mizan_exception_handler,
+    mizan_validation_handler,
+    router as mizan_router,
+)
 
 
 # ─── Fixtures ───────────────────────────────────────────────────────────────
@@ -61,6 +67,8 @@ def app():
 
     fastapi_app = FastAPI()
     fastapi_app.include_router(mizan_router, prefix="/api/mizan")
+    fastapi_app.add_exception_handler(MizanError, mizan_exception_handler)
+    fastapi_app.add_exception_handler(RequestValidationError, mizan_validation_handler)
 
     yield fastapi_app