fix(tracing): instrument FastAPI ASGI layer for inbound traceparent propagation

[STHITAPRAJNAS]

Fixes #4767.

Root cause

get_fast_api_app() calls _setup_telemetry() which correctly configures a TracerProvider and registers the W3C TraceContextTextMapPropagator. However, the FastAPI app returned to callers never had the OTel ASGI middleware applied to it. Because the traceparent header is only extracted at the ASGI boundary, every inbound HTTP request started a new trace root instead of continuing the distributed trace initiated by the upstream caller.

The existing workaround — FastAPIInstrumentor().instrument_app(app) applied manually after get_fast_api_app() returns — confirms the plumbing is otherwise correct.

What this PR does

After the FastAPI instance is created inside AdkWebServer.get_fast_api_app(), call FastAPIInstrumentor.instrument_app(app) when an OTel export pipeline is active (OTEL_EXPORTER_OTLP_* env vars or otel_to_cloud=True):

if otel_to_cloud or _otel_env_vars_enabled():
    try:
        from opentelemetry.instrumentation.fastapi import FastAPIInstrumentor
        FastAPIInstrumentor.instrument_app(app)
    except ImportError:
        logger.debug("opentelemetry-instrumentation-fastapi not installed ...")

Key design decisions:

• Conditional — only when OTel is actually configured, so local dev with the default in-memory tracer is unaffected.
• Best-effort — ImportError is caught and logged at DEBUG level; the server continues normally without the instrumentation.
• Middleware ordering — applied before CORSMiddleware and _OriginCheckMiddleware so that Starlette's reverse-registration order leaves the security wrappers outermost, and the OTel middleware extracts headers before the route handler runs.

Dependency changes

opentelemetry-instrumentation-fastapi is added to:

• otel-gcp extras — users who install google-adk[otel-gcp] now get automatic end-to-end tracing without any manual post-instrumentation step.
• test extras — required by the new unit tests.

Tests

Four new tests in tests/unittests/cli/test_fast_api.py:

Test	Verifies
test_fastapi_instrumented_when_otlp_env_var_set	instrument_app called when an OTLP env var is present
test_fastapi_instrumented_when_otel_to_cloud_enabled	instrument_app called when otel_to_cloud=True
test_fastapi_not_instrumented_without_otel_config	instrument_app NOT called when OTel is unconfigured
test_missing_fastapi_instrumentor_does_not_prevent_startup	Server starts and /health responds 200 even if the package is absent