Add server authority, action exchange, routing, usage, and schema preflight

[EricSun0218]

Summary

This PR has the exact Git tree previously reviewed in #20 and verified at a974602443214dcd409008adac43441d10a6c45f.

• add Agent Plugins 1.0 compatibility
• enforce identity-derived session and actor authorization
• add host-controlled audience projection
• add durable engine-neutral action exchange and operation ID v2
• add trusted named model routing
• expose authorized durable usage and explicit known/unknown model costs
• preflight final tool schemas before provider requests

Integrity and verification

• source and signed-branch Git tree: d5cd7900adfb6d561ed08a4bdac16fbc2d7c7b64
• every source commit in this PR is GitHub verified
• local Release build: 0 warnings, 0 errors
• 807 tests passed
• original full branch CI: https://github.com/EricSun0218/OpenGameAgent/actions/runs/31579185230
• original PR CI, CodeQL, dependency review, Unity and Godot gates passed in Add server authority, action exchange, routing, usage, and schema preflight #20