chore(example): migrate fastapi provider example

This migrates the Pact FastAPI provider from the old standalone
examples, and merges it with the new combined examples.

The consumer tests are executed first which publishes the contracts with
the broker. The provider test is then executed against the broker to
verify compliance with the published contracts.

This does, at this stage, create an unusual interdependence between
tests which typically should be avoided. I plan to fix this at a later
stage.

Signed-off-by: JP-Ellis <josh@jpellis.me>

examples/src/fastapi.py

@@ -0,0 +1,46 @@
+"""
+FastAPI provider example.
+
+This modules defines a simple
+[provider](https://docs.pact.io/getting_started/terminology#service-provider)
+with Pact. As Pact is a consumer-driven framework, the consumer defines the
+contract which the provider must then satisfy.
+
+This test assumes that the consumer has already defined a contract for the
+provider to satisfy.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from fastapi import FastAPI
+from fastapi.responses import JSONResponse
+
+app = FastAPI()
+
+"""
+As this is a simple example, we'll use a simple dict to represent a database.
+This would be replaced with a real database in a real application.
+
+When testing the provider in a real application, the calls to the database
+would be mocked out (e.g. with `unittest.mock.patch`).
+"""
+FAKE_DB: dict[int, dict[str, Any]] = {}
+
+
+@app.get("/users/{uid}")
+async def get_user_by_id(uid: int) -> dict[str, Any]:
+    """
+    Fetch a user by their ID.
+
+    Args:
+        uid: The ID of the user to fetch
+
+    Returns:
+        The user data if found, HTTP 404 if not
+    """
+    user = FAKE_DB.get(uid)
+    if not user:
+        return JSONResponse(status_code=404, content={"error": "User not found"})
+    return JSONResponse(status_code=200, content=user)