Extract tool mediation runner

[chubes4]

Summary

• Adds a public WP_Agent_Tool_Mediation_Runner facade for deterministic provider-turn tool mediation.
• Routes Conversation_Loop mediated tool-call handling through the runner without changing existing mediation behavior.
• Adds focused pure-PHP smoke coverage for direct runner normalization, execution, redaction, events, and audit output.

Testing

• php -l src/Runtime/class-wp-agent-tool-mediation-runner.php && php -l src/Runtime/class-wp-agent-conversation-loop.php && php -l tests/tool-mediation-runner-smoke.php
• php tests/tool-mediation-runner-smoke.php
• php tests/conversation-loop-tool-execution-smoke.php
• composer test
• composer phpstan

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: extracted and tested reusable tool mediation runner.