[tool] fix: forward extra_fields through ToolAgentLoop output

[cavities12]

What does this PR do?

Fixes ToolAgentLoop.run() silently dropping custom data from agent_data.extra_fields.

When tools write session data (e.g. tool_history, browse cache state) to agent_data.extra_fields during tool.execute(), this data is lost because the output is constructed with extra_fields={}. This PR replaces the empty dict with dict(agent_data.extra_fields) to shallow-copy all custom fields into the output.

turn_scores and tool_rewards are merged on top via .update(), preserving their override priority.

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Test

Unit test: Added tests/experimental/agent_loop/test_extra_fields_forwarding.py (CPU-only, runs with pytest):

Test	Verifies
test_custom_extra_fields_survive	Custom data (tool_history, session_id) appears in output
test_turn_scores_and_tool_rewards_merged	Standard fields merged alongside custom fields
test_turn_scores_overrides_custom_field	.update() correctly overrides conflicting keys
test_empty_extra_fields_still_has_turn_scores	Empty extra_fields still produces turn_scores/tool_rewards
test_shallow_copy_isolation	Modifying output doesn't mutate original dict

Production validation: Verified during multi-turn GRPO training with custom tool session data.

API and Usage Example

No API changes.

Design & Code Changes

One-line fix in tool_agent_loop.py:

-            extra_fields={},
+            extra_fields=dict(agent_data.extra_fields),

dict(...) creates a shallow copy, preventing mutations to output.extra_fields from affecting agent_data.extra_fields.

Checklist Before Submitting

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• Add unit or end-to-end test(s) to the CI workflow: tests/experimental/agent_loop/test_extra_fields_forwarding.py (CPU-only, runs with pytest).
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[gemini-code-assist]

Code Review

This pull request correctly fixes an issue where custom data in agent_data.extra_fields was being dropped. The solution, which involves creating a shallow copy of the fields, is appropriate and prevents unintended side effects. The change is accompanied by a comprehensive new test suite that thoroughly validates the fix and covers important edge cases. The implementation is clean and the tests are well-structured. No issues were found.

[ArronHZG]

#5487

I have fixed the issue in this PR; merging it will suffice. @wuxibin89