user.id silently dropped when GENERATE_CONTENT_EVENT_ONLY_EXTRA_ATTRIBUTES_CONTEXT_KEY is missing

## Summary

When `opentelemetry-instrumentation-google-genai` is installed (Vertex Agent Engine / Gemini path), ADK collects `session.user_id` into `log_only_extra_attributes` but **never emits `user.id`** on spans or GenAI log events. The hand-off depends on a context key that the instrumentation package does not export (through `1.0b0`). ADK catches the import failure and silently continues.

This contradicts the documented expectation that `user.id` appears on `generate_content` telemetry when content capture is enabled (`OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT`, with `OTEL_SEMCONV_STABILITY_OPT_IN=gen_ai_latest_experimental`).

## Environment

- `google-adk` 2.1.0 and 2.4.0 (same code path; also present on `main` as of 2026-07-10)
- `opentelemetry-instrumentation-google-genai` 0.7b1 and **1.0b0**
- Vertex AI Agent Engine + local runs with `GoogleGenAiSdkInstrumentor` active

## Call chain

```text
Runner.run_async(user_id=...)
→ Session.user_id / InvocationContext
→ use_inference_span(...)
log_only_common_attributes[USER_ID] = session.user_id
→ _use_extra_generate_content_attributes(..., log_only_extra_attributes=...)
try:
from opentelemetry.instrumentation.google_genai import
GENERATE_CONTENT_EVENT_ONLY_EXTRA_ATTRIBUTES_CONTEXT_KEY
except (ImportError, AttributeError):
pass # ← user.id discarded with no warning
→ GoogleGenAiSdkInstrumentor
only reads GENERATE_CONTENT_EXTRA_ATTRIBUTES_CONTEXT_KEY
→ generate_content {model} span / Cloud Trace ← no user.id
```

Relevant code: `src/google/adk/telemetry/tracing.py` → `_use_extra_generate_content_attributes`.

## Root cause

`opentelemetry-instrumentation-google-genai` exports only:

- `GENERATE_CONTENT_EXTRA_ATTRIBUTES_CONTEXT_KEY`

It does **not** export `GENERATE_CONTENT_EVENT_ONLY_EXTRA_ATTRIBUTES_CONTEXT_KEY` (proposed in [open-telemetry/opentelemetry-python-genai#24](https://github.com/open-telemetry/opentelemetry-python-genai/pull/24), closed without merge).

ADK puts `user.id` only in the EVENT_ONLY channel (intentionally “log-only” / PII-sensitive), so there is no fallback onto the working span extras key.

## Expected

`user.id` should appear on GenAI telemetry (at least on allowlisted log/events, and ideally discoverable in Cloud Trace) when `session.user_id` is set and content capture is enabled.

## Actual

`user.id` is absent from every span/event produced via the instrumentor path. Failure is silent (`except: pass`).

## Suggested fix (ADK-side, until instrumentation ships the key)

In `_use_extra_generate_content_attributes`, when the EVENT_ONLY import fails, merge `log_only_extra_attributes` into the existing span extras dict (`GENERATE_CONTENT_EXTRA_ATTRIBUTES_CONTEXT_KEY`) and/or log a clear warning that `user.id` could not be propagated.

Trade-off: putting `user.id` on spans is slightly broader than “event-only”, but is far better than silently dropping identity for all Agent Engine Trace users. Longer-term, either:

1. instrumentation exports and honors the EVENT_ONLY key, or
2. ADK adopts a LogRecordProcessor-based propagation (related attempt: #6217, not on a released tag / `main` as of this report).

## Repro

1. Install ADK ≥ 2.1 + `opentelemetry-instrumentation-google-genai`.
2. Enable instrumentor; set `OTEL_SEMCONV_STABILITY_OPT_IN=gen_ai_latest_experimental` and `OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT=EVENT_ONLY`.
3. Run an agent with a non-default `user_id`; export spans (e.g. `InMemorySpanExporter`).
4. Observe `generate_content …` spans: agent/conversation attrs present, **`user.id` missing**.

We verified a one-line-style fallback (merge into span extras on ImportError) restores `user.id` on `generate_content` spans in production Cloud Trace.

Happy to open a PR with that fallback if maintainers prefer an ADK-side mitigation while the instrumentation gap remains.