Add test for start_observe with external parent span

[Copilot]

Root-span detection now treats start_observe spans as Basalt roots when a non-Basalt parent span exists (e.g., from FastAPI/httpx instrumentation). Existing tests didn't cover this case.

Changes

• Added test_start_observe_with_external_parent_span: Verifies that start_observe correctly establishes a Basalt root when an external OpenTelemetry span exists as parent but ROOT_SPAN_CONTEXT_KEY is not set

The test simulates the scenario where framework instrumentation creates a parent span, then asserts:

• basalt.root attribute is set to True
• ROOT_SPAN_CONTEXT_KEY is attached to the context

# Simulate external instrumentation (FastAPI, httpx, etc.)
external_tracer = provider.get_tracer("external.instrumentation")
external_span = external_tracer.start_span("external_http_request")
external_context = trace.set_span_in_context(external_span)
otel_context.attach(external_context)

# start_observe should treat this as a new Basalt root
with StartObserve(name="test_basalt_root", feature_slug="test_feature") as span:
    assert span._span.attributes.get("basalt.root") is True
    assert otel_context.get_value(ROOT_SPAN_CONTEXT_KEY) is not None

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