Define server tool declaration contract

[chubes4]

Summary

• Adds a canonical WP_Agent_Tool_Declaration::normalizeForServer() contract for host/server-mediated tools while keeping normalize() strict for client runtime declarations.
• Routes source registry, conversation-loop declarations, and provider-turn request declarations through the canonical request/server normalizer.
• Documents the server declaration envelope and adds smoke coverage for server declarations, legacy client catalog defaults, registry normalization, and provider-turn normalization.

Fixes #282.

Testing

• php tests/tool-runtime-smoke.php
• php tests/tool-source-registry-smoke.php
• php tests/provider-turn-adapter-smoke.php
• php tests/conversation-loop-tool-execution-smoke.php
• php tests/tool-policy-contracts-smoke.php && php tests/action-policy-resolver-smoke.php && php tests/tool-tier-resolver-smoke.php
• php tests/pre-execute-approval-smoke.php
• php tests/spin-detector-smoke.php
• php tests/identical-failure-tracker-smoke.php
• composer smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted and tested the canonical server-side tool declaration contract, docs, and smoke coverage for Chris to review.