fix(starlette): handle FastAPI 0.137 _IncludedRouter in route name resolution

[HosseinNejatiJavaremi]

Summary

Fixes #2681

FastAPI 0.137.0 changed app.routes to include _IncludedRouter wrapper objects for routes added via include_router(). These wrappers do not expose a .path attribute, but ElasticAPM._get_route_name() accessed route.path unconditionally on full and partial matches, causing AttributeError and 500 responses on every HTTP request.

This PR:

• Adds _flatten_routes() using FastAPI's public iter_route_contexts() (>= 0.137.2) with effective_route_contexts() fallback for 0.137.0/0.137.1
• Only flattens when _IncludedRouter nodes are present, preserving existing Starlette Mount behavior
• Guards partial-match .path access with getattr()
• Adds FastAPI regression tests and pins fastapi>=0.137.2 in reqs-starlette-newest.txt

Prior art: open-telemetry/opentelemetry-python-contrib#4700

Reproduction (before fix)

1. Install fastapi>=0.137 and elastic-apm
2. Create a FastAPI app with app.include_router(...)
3. Add app.add_middleware(ElasticAPM, client=apm_client)
4. Send any request → AttributeError: '_IncludedRouter' object has no attribute 'path'

Test plan

• pytest tests/contrib/asyncio/starlette_tests.py tests/contrib/asyncio/fastapi_tests.py -m starlette — 30 passed locally
• Existing Starlette mount/trailing-slash tests still pass
• New tests: included router GET succeeds, transaction name uses composed template, OPTIONS partial match does not crash

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