feat(ext): fastapi network context

[ChickenBenny]

What I did

Added ape.ext.fastapi — a purpose-built FastAPI integration module that provides
a network_context() decorator for managing Ape network contexts in route handlers.

This is the first integration under the new ape.ext package, which is intended
to house future framework integrations (e.g. Flask, Django).

fixes: #2761

How I did it

• Created ape/ext/ as a new package for external framework integrations
• Implemented @network_context() in ape/ext/fastapi.py
• The decorator automatically injects ecosystem and network as FastAPI
  Query parameters by patching wrapper.__signature__ at decoration time —
  so route handlers do not need to declare them explicitly
• If the handler does declare ecosystem/network, they are forwarded as normal
• fastapi is kept as an optional dependency, imported lazily inside the decorator
  to avoid making it a hard requirement for all Ape users
• Only synchronous routes are supported for now, as async network context
  is not yet available in Ape

How to verify it

Install fastapi and run:

pytest tests/functional/test_ext_fastapi.py -v

Expected: 9 passed.

Example usage:

from ape.ext.fastapi import network_context

@app.post("/faucet/{token}")
@network_context()
def send_token(token: str, receiver: str, amount: int) -> str:
    # POST /faucet/USDC?ecosystem=ethereum&network=mainnet
    # Ape is automatically connected to the requested network
    ...

Checklist

• All changes are completed
• Change is covered in tests
• Documentation is complete

[fubuloubu]

I think that the tests might not work without adding fastapi to test dependencies