feat(core): support lazy tool search registration

[fcoury-oai]

Why

tool_search needs a reusable way to surface tools discovered after a turn starts and dispatch a returned MCP tool immediately. This infrastructure should be reviewable separately from the MCP startup policy that supplies the loaders.

What Changed

• Add a lazy tool registry alongside the static registry.
• Allow tool_search to refresh optional MCP search entries through injected loaders.
• Register only returned lazy MCP handlers so they can be called during the same turn.
• Rebuild normalized MCP entries together when inventory changes.
• Reject lazily loaded tools shadowed by existing static handlers while preserving equivalent static MCP handlers.

This layer is dormant until a caller supplies lazy MCP loaders.

Stack

1. #25212 background MCP startup status
2. #25213 atomic startup header handoff
3. #25214 explicit MCP dependency readiness
4. feat(core): support lazy tool search registration #25211 lazy tool-search registration (this PR)
5. #24987 pending MCP inventory integration

How to Test

Targeted tests:

• just test -p codex-core lazy_mcp_search

The focused run covers same-turn dispatch registration, pending-loader avoidance when existing results match, ready-tool refresh, normalized-name replacement, and shadow handling.