Handle Cerebras reasoning replay fields

[NubsCarson]

Summary

• strip Cerebras/OpenAI-compatible reasoning_content and reasoning_details replay fields before sending prior assistant messages back to Cerebras-compatible endpoints
• preserve useful reasoning context by folding replayed reasoning into assistant text instead of dropping it entirely
• keep DeepSeek/Kimi/GLM-style reasoning replay behavior for providers that accept those fields
• add coverage for native Cerebras, custom OpenAI-compatible Cerebras base URLs, gpt-oss reasoning replay, and serialized top-level reasoning replay

This is the OpenCode-side fix needed for the gpt-oss/Cerebras sub-agent path validated in elizaOS/eliza#7716. Eliza should continue to use the vendored OpenCode source; no fork-only submodule pointer was added to the Eliza PR.

Verification

Run in packages/opencode:

• bun test test/provider/transform.test.ts test/provider/provider.test.ts --timeout 30000 — 309 pass, 0 fail, 600 assertions
• bun run typecheck
• git diff --check origin/dev...HEAD
• hygiene scan over the PR diff for personal paths/domains, secrets placeholders, debug logs, and hack markers returned no matches

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

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• Commit unit tests in branch nubs/cerebras-reasoning-replay-clean-20260515

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