Python: parse structured response value from final message

[liuzemei]

Motivation and Context

Fixes #6366.

When an agent response contains intermediate messages and a final structured JSON payload, AgentResponse.value currently parses self.text, which concatenates all message text. That can join intermediate tool/context JSON with the final response JSON and cause ValidationError / JSONDecodeError from trailing characters.

Description

• Parse structured ChatResponse.value and AgentResponse.value from the last non-empty message text instead of the public concatenated .text value.
• Keep .text behavior unchanged for callers that want the full concatenated response text.
• Add regression coverage for Pydantic response formats and JSON schema mapping response formats.

Testing

uv run --group dev pytest packages/core/tests/core/test_types.py -q -k 'final_message or empty_text or invalid_schema or chat_response_with_format or mapping_response_format'\nuv run pytest packages/core/tests/core/test_types.py -q\nuv run pytest packages/core/tests/core/test_middleware.py -q\nuv run pytest packages/core/tests/workflow/test_workflow.py -q -k 'AgentResponse or response or output'\nuv run ruff format --check packages/core/agent_framework/_types.py packages/core/tests/core/test_types.py\nuv run ruff check packages/core/agent_framework/_types.py packages/core/tests/core/test_types.py\ngit diff --check\n```\n\n### Contribution Checklist\n\n- [x] The code builds clean without any errors or warnings\n- [x] The PR follows the Contribution Guidelines\n- [x] All unit tests pass, and I have added new tests where possible\n- [ ] **Is this a breaking change?** If yes, add "[BREAKING]" prefix to the title of the PR.

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[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR updates structured-output parsing for ChatResponse.value and AgentResponse.value to parse only the final non-empty message (ignoring earlier messages), and adds regression tests for multi-message responses.

Changes:

• Add _last_non_empty_message_text() helper to select the final non-empty message text.
• Update ChatResponse.value / AgentResponse.value to parse structured output from the final message only.
• Add tests asserting intermediate messages are ignored when parsing structured output.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
python/packages/core/agent_framework/_types.py	Parse structured output from the last non-empty message instead of concatenated response text.
python/packages/core/tests/core/test_types.py	Add tests validating structured parsing ignores intermediate messages for both response types.