fix(anthropic): keep tools array when toolChoice is 'none'

[giulio-leone]

Fixes #12378

Problem

When toolChoice is 'none', the Anthropic provider strips both tools and tool_choice from the API request. This causes empty content response when conversation history contains tool_use/tool_result blocks.

Fix

• Changed 'none' case to return tools array and toolChoice: { type: 'none' } instead of undefined for both
• Added 'none' to AnthropicToolChoice type union
• Updated tests

[giulio-leone]

Intervention note for this PR:

Current blocker appears to be Vercel deployment authorization, not code/test correctness.

Observed pattern:

• Vercel check fails with: "Authorization required to deploy."
• Other checks (e.g., Socket Security / Vercel Agent Review) are passing.

Recommended unblock sequence:

1. Complete the Vercel Git authorization flow from the failing check link.
2. Re-run checks on this PR.
3. If Vercel passes, this PR should move forward with normal review.

If useful, I can follow up with a PR-by-PR status sweep after authorization is completed.

[giulio-leone]

Ready for review — fixes Anthropic provider failing when toolChoice is 'none' by preserving the tools array. Includes regression test.

[giulio-leone]

This PR is ready for review — all CI checks pass, no merge conflicts, and all review threads have been resolved. Ready to merge when approved. 🚀

[giulio-leone]

Hi! Gentle ping — this PR is rebased on main, CI passes, and ready for review. Happy to address any feedback. Thanks!

[giulio-leone]

Closing to reduce PR volume. Happy to resubmit if useful.