feat(otel): emit gen_ai.response.finish_reasons on the agent span

[Copilot]

✨ Enhancement

What does this improve?
readAgentRuntimeMetrics already parsed the agent's stdio result JSON line but discarded stop_reason. Without it, gen_ai.response.finish_reasons was never set on the gh-aw.<job>.agent span, making it impossible to distinguish successful completions from silent max_tokens truncations in any OTel backend.

Implementation approach:

• buildArrayAttr helper — new function that produces a properly-encoded OTLP arrayValue attribute (required for gen_ai.response.finish_reasons which is string[] in the GenAI semconv)
• readAgentRuntimeMetrics — extracts stop_reason from the result JSON alongside the existing num_turns / total_cost_usd; stored as stopReason on AgentRuntimeMetrics
• sendJobConclusionSpan — pushes gen_ai.response.finish_reasons: [stopReason] onto agentAttributes when present; omitted when the field is absent (e.g. cancelled/timed-out runs)

The resulting agent span now enables backend queries like:

# Detect silently truncated runs
{ gen_ai.response.finish_reasons =~ "max_tokens" }

Docs: gen_ai.response.finish_reasons added to the agent span attributes table in frontmatter.md.

[Copilot]

Pull request overview

Adds support for emitting gen_ai.response.finish_reasons on the dedicated agent span by extracting stop_reason from the agent stdio result line and encoding it as an OTLP arrayValue attribute, enabling observability backends to distinguish normal completions from truncations like max_tokens.

Changes:

• Add buildArrayAttr() helper to encode string[] OTLP attributes via arrayValue.
• Extend readAgentRuntimeMetrics() to capture stop_reason and attach it to the agent span as gen_ai.response.finish_reasons.
• Update docs and add unit tests verifying the attribute is included/omitted appropriately.

Show a summary per file

File	Description
docs/src/content/docs/reference/frontmatter.md	Documents gen_ai.response.finish_reasons as an agent span attribute.
actions/setup/js/send_otlp_span.cjs	Parses stop_reason and emits gen_ai.response.finish_reasons on the agent span using a new array-attribute helper.
actions/setup/js/send_otlp_span.test.cjs	Adds coverage for inclusion/omission of gen_ai.response.finish_reasons based on presence of stop_reason.

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• Files reviewed: 3/3 changed files
• Comments generated: 0