[FastAPI] Add route resolver

[theDakshJaitly]

Target branch: main

Dependency

The Python code-graph extractor was merged in #103 and is available on main.
This issue is not blocked by another language extractor.

Goal

Add a bounded FastAPI framework resolver that connects decorator-defined HTTP
routes to their endpoint functions.

The initial implementation should cover statically recognizable routes without
evaluating application code or attempting to reconstruct the complete runtime
router.

Scope

• Detect FastAPI from declared project dependencies such as pyproject.toml
  or requirements files.
• Recognize literal route decorators including:
  • @app.get(), @app.post(), @app.put(), @app.patch()
  • @app.delete(), @app.options(), @app.head()
  • Equivalent decorators on APIRouter instances such as @router.get()
• Support both synchronous and asynchronous endpoint functions.
• Emit one stable route node for each recognized HTTP method and literal path.
• Emit a function_ref from the route node to the decorated endpoint.
• Resolve the reference only when the matching same-file function or method is
  unambiguous.
• Preserve parameterized paths such as /users/{user_id}.
• Leave missing or ambiguous handlers unresolved rather than guessing.

Required changes

• Add src/graph/resolution/frameworks/fastapi.ts implementing
  FrameworkResolver.
• Register and export it from
  src/graph/resolution/frameworks/index.ts.
• Add a minimal FastAPI fixture under
  src/graph/__tests__/fixtures/.
• Add focused detection, extraction, resolution, and ambiguity tests.

Acceptance criteria

• Positive and negative FastAPI detection are tested.
• Both FastAPI application and APIRouter decorators are covered.
• Tests cover multiple HTTP methods and a parameterized path.
• Both def and async def endpoints are covered.
• Multiple decorators on one endpoint produce the expected route nodes.
• Route nodes use stable IDs and Python as their language.
• Same-file endpoints resolve only when unambiguous.
• Missing and ambiguous handlers remain unresolved.
• The resolver is registered in the framework registry.
• npm run typecheck, npm test, and npm run build pass.
• No identity, reconciliation, schema, or drift-semantics changes are included.

Out of scope

• Composing prefixes across include_router() calls.
• Runtime dependency injection or middleware analysis.
• WebSocket routes, mounted applications, and event handlers.
• Dynamic or computed route paths.
• Evaluating Python modules or importing the application.

Contributor references

• Extending the code graph
• src/graph/resolution/frameworks/express.ts
• src/graph/__tests__/resolver-express.test.ts

Contributor workflow

git switch main
git pull
git switch -c feat/fastapi-route-resolver

Open the pull request with main as the base branch.