Expose mediated tool hook surfaces

[chubes4]

Summary

• Adds a host-neutral agents_api_pre_tool_call_decision filter around mediated tool preflight while preserving the existing pre_tool_mediator option path.
• Adds explicit pending preflight decisions that normalize to the canonical runtime-tool pending request/status and stop the loop as incomplete.
• Adds post_tool_result_diagnostics plus agents_api_mediated_tool_result_diagnostics for audit-oriented host diagnostics without storing raw tool parameters.

Closes #259.

Testing

• php tests/conversation-loop-tool-execution-smoke.php
• composer phpstan
• composer smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Implemented the mediated-loop hook surfaces, pending external-tool decision handling, diagnostics seam, and smoke test coverage; Chris remains responsible for review and merge.