fix: preserve normalized pending mediation requests

[chubes4]

Summary

• make pending pre-tool mediation normalization idempotent when an option-provided decision passes unchanged through agents_api_pre_tool_call_decision
• preserve the canonical runtime-tool request and its host-provided identity when re-normalizing the pending tool-result envelope
• add focused coverage for a no-op filter, custom request ID, and runtime-tool store

Root cause

pre_tool_mediation_decision() normalized the option mediator's pending decision before applying agents_api_pre_tool_call_decision, then normalized the filter result again. On that second pass, normalize_pre_tool_mediation_decision() treated the already-normalized decision['result'] tool-result envelope as though it were the pending request itself. Because the actual request was nested under result.runtime_tool_request, request normalization could not see the host-provided request_id and generated a replacement hash ID.

The pending branch now recognizes an already-normalized tool-result envelope, unwraps its canonical runtime_tool_request, and retains the envelope's runtime, metadata, and error fields. Existing compatibility is preserved: explicit top-level runtime_tool_request remains first priority, the request alias remains second, and legacy raw requests supplied through result continue to normalize as before.

Verification

• php tests/conversation-loop-tool-execution-smoke.php (123 assertions passed)
• php -l src/Runtime/class-wp-agent-conversation-loop.php (no syntax errors)
• php -l tests/conversation-loop-tool-execution-smoke.php (no syntax errors)
• composer phpstan (no errors)
• git diff --check (clean)

Fixes #521