Python: fix(copilot): propagate usage details, finish_reason, and model from SDK events

[droideronline]

Closes #6930

Summary

The Copilot SDK emits an ASSISTANT_USAGE event after every model turn that carries full input/output token counts, cache tokens, reasoning tokens, finish reason (reason), and model name. Previously all of this metadata was discarded and AgentResponse.usage_details, AgentResponse.finish_reason, and model info were always missing.

Problem

When calling agent.run() with the GitHub Copilot agent, AgentResponse.usage_details and AgentResponse.finish_reason are always None.

Direct SDK invocation emits:

SessionEvent(type=ASSISTANT_USAGE, data=AssistantUsageData(
    model="claude-sonnet-4-5",
    input_tokens=120, output_tokens=40,
    cache_read_tokens=5, reason="stop"
))

Agent Framework before this fix:

response.usage_details  # None ❌
response.finish_reason  # None ❌

Agent Framework after this fix:

response.usage_details  # {"input_token_count": 120, "output_token_count": 40, "total_token_count": 160, "cache_read_input_token_count": 5} ✅
response.finish_reason  # "stop" ✅
response.additional_properties["model"]  # "claude-sonnet-4-5" ✅

Changes

python/packages/github_copilot/agent_framework_github_copilot/_agent.py

1. Adds UsageDetails to imports from agent_framework.
2. Adds _parse_copilot_usage() — converts the SDK event data payload to UsageDetails using getattr for forward compatibility (works with the Data shim in SDK v1.0.2 and with AssistantUsageData in newer SDK releases).
3. Non-streaming (_run_impl) — subscribes an extra event handler before send_and_wait to capture ASSISTANT_USAGE events. After send_and_wait completes, the collected data populates AgentResponse.usage_details, finish_reason, and model name in additional_properties. Falls back to output_tokens on ASSISTANT_MESSAGE data when no dedicated usage event arrives (older CLI versions).
4. Streaming (_stream_updates) — handles SessionEventType.ASSISTANT_USAGE in the event handler, yielding an AgentResponseUpdate with Content.from_usage() payload so _process_update() aggregates it into AgentResponse.usage_details, and propagates finish_reason.

python/packages/github_copilot/tests/test_github_copilot_agent.py

Adds 8 tests in TestGitHubCopilotAgentUsageMetadata:

• test_run_propagates_usage_details_from_assistant_usage_event
• test_run_propagates_finish_reason_from_assistant_usage_event
• test_run_fallback_to_output_tokens_when_no_usage_event
• test_run_no_usage_when_no_usage_event_and_no_output_tokens
• test_run_propagates_model_name_in_additional_properties
• test_run_streaming_propagates_usage_details
• test_run_streaming_propagates_finish_reason
• test_run_streaming_no_usage_event_no_crash

Testing

All 128 existing tests pass. 8 new tests added (136 total).

[Copilot]

Pull request overview

This PR fixes missing response metadata in the Python GitHub Copilot agent by propagating SDK-emitted usage and completion information into AgentResponse / streaming updates, aligning Copilot behavior with other chat clients in the framework.

Changes:

• Add parsing for ASSISTANT_USAGE event payloads into UsageDetails (token counts, cache tokens, reasoning tokens).
• Populate non-streaming AgentResponse.usage_details, finish_reason, and model name (via additional_properties).
• In streaming mode, emit usage updates so standard _process_update() aggregation populates final usage/finish metadata, and add targeted tests for both modes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
python/packages/github_copilot/agent_framework_github_copilot/_agent.py	Capture Copilot SDK usage events and map them into Agent Framework response metadata for streaming and non-streaming runs.
python/packages/github_copilot/tests/test_github_copilot_agent.py	Add test coverage ensuring usage details, finish reason, and model metadata are propagated correctly (including fallback and no-crash cases).

[droideronline]

@moonbox3 @eavanvalkenburg - kindly take a look when you get a chance. Thanks!