fix: publish the endpoint guard through aiToolkit's public barrel

[Bryandero98]

Summary

• evaluateSecretEndpoint/assertSecretEndpoint (the SSRF/key-exfiltration guard for provider endpoint URLs) lived only at the toolkit's private internal/endpointGuard.js path, yet six server/services/* modules imported it from there directly.
• Promotes both as named exports of server/lib/aiToolkit/index.js and repoints every service import at the public barrel. internal/endpointGuard.js stays the sole implementation — this is an API-path correction, not a policy change.
• Documents the new public surface in server/lib/README.md.
• The guard's own contract test now imports through the barrel, so it exercises what services actually call.
• Fixed a stale test mock (visionTest.frameGuard.test.js) still pinned to the old internal path, found while grepping the whole repo for any remaining reference — it would have silently stopped intercepting the guard call once the production import moved.

Test plan

• npx vitest run lib/aiToolkit/internal/endpointGuard.test.js services/aiProvider.test.js services/askService.test.js services/localLlmPlayground.test.js services/visionTest.test.js services/voice/llm.test.js services/insightsService.test.js services/visionTest.frameGuard.test.js — all pass
• Repo-wide grep confirms no remaining production import of aiToolkit/internal/endpointGuard.js outside the toolkit's own internal modules (providers.js, runner.js, validation.js)
• Sanity-checked the barrel resolves with no circular-import error

Fixes #5619

[Bryandero98]

Thanks for the well-specified issue — the reuse search and the exact call-site list made this a clean, mechanical fix to verify end-to-end. Happy to adjust anything or take on more of this module-hygiene series if useful.