fix(provider): detect configured Cerebras endpoints

[NubsCarson]

Issue for this PR

Follow-up to #2. Custom OpenAI-compatible provider configs can point at Cerebras through options.baseURL without declaring a provider/model api URL, which means the Cerebras reasoning replay normalizer cannot reliably identify the resolved endpoint.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This PR makes configured custom endpoints visible to provider transforms and tightens Cerebras endpoint detection:

• copies a custom provider's configured endpoint or baseURL into model.api.url when no model/provider api field is declared
• detects Cerebras-compatible endpoints by parsing the configured URL hostname, not by matching arbitrary URL text
• keeps the existing Cerebras reasoning replay behavior from Handle Cerebras reasoning replay fields #2 intact

This lets a user configure an OpenAI-compatible provider with baseURL: "https://api.cerebras.ai/v1" and still get the same reasoning replay cleanup as the dedicated @ai-sdk/cerebras provider.

How did you verify your code works?

• bunx prettier --write src/provider/provider.ts
• bun test --timeout 30000 test/provider/provider.test.ts test/provider/transform.test.ts -> 331 passed, 0 failed
• bun run typecheck was checked; it still reports two current-dev baseline errors unrelated to this patch:
  • src/provider/transform.ts(19,38) missing OPENCODE_EXPERIMENTAL_OUTPUT_TOKEN_MAX flag typing
  • test/acp/event-subscription.test.ts(11,30) missing ../../src/project/with-instance

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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✨ Finishing Touches

🧪 Generate unit tests (beta)

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• Commit unit tests in branch nubs/cerebras-compatible-endpoint-detection-20260520

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[github-actions]

Hey! Your PR title Detect configured Cerebras-compatible endpoints doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[NubsCarson]

Status refresh (2026-05-22): the branch is still current against origin/dev (0 behind / 1 ahead) and the scoped provider tests for this patch pass locally.

Verified in a fresh scratch checkout of this PR:

• bun install completed cleanly
• bun --cwd packages/opencode test test/provider/provider.test.ts test/provider/transform.test.ts -> 331 pass, 0 fail

I also checked the broad root typecheck failure against plain origin/dev. bun run typecheck fails the same way on base, before this PR's single commit, with existing opencode package issues such as:

• src/bus/global.ts typed EventEmitter override
• missing Flag.OPENCODE_EXPERIMENTAL_OUTPUT_TOKEN_MAX typing
• @lydell/node-pty declaration resolution
• missing test/acp/.../with-instance import

So the PR-specific provider/transform coverage is passing; the red broad/cancelled checks do not appear caused by the endpoint-detection diff.