feat: migrate Cerebras provider to AI SDK

[daniel-lxs]

Summary

Migrate the Cerebras provider from manual fetch-based implementation (362 lines) to use OpenAICompatibleHandler with Vercel AI SDK (~70 lines).

Changes

• Rewrote src/api/providers/cerebras.ts using AI SDK pattern
• Updated tests to match AI SDK streaming behavior (tool-call events now ignored)
• Preserved X-Cerebras-3rd-Party-Integration: roocode attribution header

Removed (AI SDK handles automatically)

• Manual fetch and SSE parsing
• TagMatcher for <think> tags (AI SDK emits reasoning-delta)
• Tool schema stripping (minItems/maxItems)
• Manual token estimation

Testing

• All 17 Cerebras tests pass
• Full test suite passes (5316 tests)

Fixes EXT-690

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Important

Migrates Cerebras provider to AI SDK, simplifying implementation and updating tests for new streaming behavior.

• Behavior:
  • Migrates CerebrasHandler in cerebras.ts to use OpenAICompatibleHandler with Vercel AI SDK.
  • Preserves X-Cerebras-3rd-Party-Integration: roocode header.
  • Updates tests in cerebras.spec.ts to match AI SDK streaming behavior, ignoring tool-call events.
• Removed:
  • Manual fetch and SSE parsing.
  • TagMatcher for <think> tags.
  • Tool schema stripping (minItems/maxItems).
  • Manual token estimation.
• Testing:
  • All 17 Cerebras tests pass.
  • Full test suite passes (5316 tests).
• Dependencies:
  • Adds @ai-sdk/cerebras to package.json.

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[roomote]

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Review complete. All previous issues have been addressed.

• Move @ai-sdk/cerebras from devDependencies to dependencies in src/package.json (it's used in production code)

Previous reviews

• a212574: Review #1
• 1976734: Review #2

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