fix: export thoughts_token_count to OpenTelemetry trace spans

[brucearctor]

Description

Fixes #4829

ADK's OpenTelemetry tracing does not export thoughts_token_count to span attributes. When using Gemini models with ThinkingConfig, the usage_metadata in LlmResponse correctly contains thoughts_token_count, but this field is never written to spans by trace_generate_content_result() or trace_inference_result().

Interestingly, trace_call_llm() already exports this field (as gen_ai.usage.experimental.reasoning_tokens). This PR adds the same export to the two remaining functions that were missing it.

Changes

src/google/adk/telemetry/tracing.py

• Added thoughts_token_count → gen_ai.usage.experimental.reasoning_tokens span attribute export in trace_generate_content_result() (~line 746)
• Added the same export in trace_inference_result() (~line 789)
• Uses the same try/except AttributeError guard pattern as trace_call_llm() for backward compatibility with older SDK versions

tests/unittests/telemetry/test_spans.py

• Added test_trace_inference_result_with_thinking_tokens — verifies the attribute is exported when thoughts_token_count is non-None
• Added test_trace_inference_result_without_thinking_tokens — verifies no attribute is set when thoughts_token_count is None

Testing Plan

Unit Tests

All 23 telemetry tests pass:

$ pytest tests/unittests/telemetry/test_spans.py -v
23 passed in 1.08s

New tests specifically verify:

1. thoughts_token_count=50 → span attribute gen_ai.usage.experimental.reasoning_tokens=50 is set
2. thoughts_token_count=None → no gen_ai.usage.experimental.reasoning_tokens attribute on span

Verification

Before fix — Event.usage_metadata.thoughts_token_count is non-zero but Cloud Trace spans only show gen_ai.usage.input_tokens and gen_ai.usage.output_tokens.

After fix — gen_ai.usage.experimental.reasoning_tokens appears alongside the existing token attributes in all three tracing functions.

[gemini-code-assist]

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