fix(opencode): retry network errors wrapped by providers - #53397

Issue for this PR

Closes #53396

Type of change

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

What does this PR do?

Retry only looks at the error message, so a network error wrapped by a provider (e.g. "Cursor HTTP/2 connection failed" with ENOTFOUND on .cause) isn't retried and the session stops.

• fromError walks the cause chain for a network code (ENOTFOUND, ECONNREFUSED, ETIMEDOUT, ...) and returns a retryable APIError with that code in metadata. It stops at an error marked transient: false / isRetryable: false, so a provider that already gave up isn't retried again.
• The same code is read from the cause of an AI SDK APICallError. Direct providers (Anthropic, OpenAI) wrap a failed fetch as Cannot connect to API: ... with the socket error on cause; before, those got only the default 5 retries.
• Bun's fetch uses its own codes (ConnectionRefused, FailedToOpenSocket, ConnectionClosed, ConnectionResetByPeer, ConnectionTimeout) and messages (Unable to connect. Is the computer able to access the url?, Was there a typo in the url or port?). Both are now recognized.
• Network errors get 720 retries instead of 5. With the existing backoff (capped at 30s) that's about 6 hours, so an outage is waited out instead of failing the turn; the session shows the retry status throughout. Other errors keep 5.
• The code list lives in retry.ts. I removed its unused MessageV2 import so message-v2.ts can import it without a cycle.

How did you verify your code works?

• New tests: a wrapped ENOTFOUND is retryable, a wrapped error marked transient: false is not, and the policy allows 24 retries for network errors.
• bun test test/session/retry.test.ts test/session/message-v2.test.ts and bun run typecheck pass.
• To reproduce: use cursor-opencode-provider, start a prompt, and turn off Wi-Fi. Before this change the session stops with "Cursor HTTP/2 connection failed". After it, the session shows retry status and continues once you're back online.

Screenshots / recordings

N/A

Checklist

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