fix: #3288 normalize RunState guardrail payloads

[Aphroq]

Summary

• Normalize RunState guardrail result payloads so guardrail outputInfo, output guardrail agentOutput, and tool guardrail outputInfo remain JSON-compatible.
• Add regression coverage for non-JSON guardrail payloads using datetime values in input/output guardrails and tool input/output guardrails.

Test plan

• uv run pytest tests/test_run_state.py -k guardrail_results_to_string_normalizes_non_json
• uv run pytest tests/test_run_state.py -k guardrail_results
• bash .agents/skills/code-change-verification/scripts/run.sh

Issue number

Closes #3288

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation ()
• I've run make lint and make format
• I've made sure tests pass