Annotate vMCP optimizer discovery tool

[Whxuan0701]

Summary

The vMCP optimizer exposes find_tool as a local tool-discovery operation, but its MCP definition currently carries no behavioral hints. Clients cannot distinguish this safe discovery step from a tool that may mutate external state, which limits safe retry and tool-selection behavior in Agent workflows.

• Add MCP annotations to find_tool: read-only, non-destructive, idempotent, and closed-world.
• Leave call_tool unannotated because it can proxy arbitrary backend side effects.

Part of #4357

Type of change

• Bug fix
• New feature
• Refactoring (no behavior change)
• Dependency update
• Documentation
• Other (describe):

Test plan

• Unit tests (task -t Taskfile.focused.yml test, using the repository's -race and linker settings for ./pkg/vmcp/session/optimizerdec and ./pkg/vmcp/conversion)
• E2E tests (task test-e2e)
• Linting (task lint-fix)
• Manual testing (describe below)

API Compatibility

• This PR does not break the v1beta1 API.

Changes

File	Change
pkg/vmcp/session/optimizerdec/decorator.go	Add behavioral annotations to find_tool.
pkg/vmcp/session/optimizerdec/decorator_test.go	Assert the discovery tool annotations and preserve the unannotated call_tool contract.

Does this introduce a user-facing change?

Yes. MCP clients that consume tool annotations can now identify find_tool as a safe, retryable local discovery operation.

Special notes for reviewers

The annotation is deliberately limited to find_tool. call_tool dispatches arbitrary backend tools, so asserting read-only or non-destructive behavior there would be incorrect.