Suspend client tools in the default chat runtime

[chubes4]

Summary

• accept request-scoped client tool declarations only through trusted server filters
• route executor: client calls to canonical runtime_tool_pending outcomes
• declare pending status/request/outcome in the canonical chat output schema
• optionally persist pending requests through the host runtime-tool store for generic submission and continuation
• preserve request-injection stripping, policy narrowing, alias safety, and host ability execution

Verification

• strict Composer validation
• default chat handler and frontend REST focused tests
• full composer smoke
• PHPStan
• git diff --check

Closes #512.

AI assistance

OpenAI openai/gpt-5.6-sol via OpenCode general coding agents implemented, security-reviewed, and verified the client-tool suspension contract. Chris Huber reviewed and is responsible for every line.