fix(opencode): retry transient network errors instead of surfacing as terminal with raw content

[Sylchi]

Issue for this PR

Closes #31133, closes #20822, closes #15350, closes #21893

Type of change

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

What does this PR do?

When a network request drops mid-flight (ECONNRESET, ECONNREFUSED, fetch failure, premature close, etc.), the error was classified as NamedError.Unknown and never retried — leaving the raw error body (potentially an HTML error page or thousands of characters) dumped on screen.

Two changes:

1. Transport error detection in MessageV2.fromError() — Added isTransientNetworkError() helper that catches SystemError codes (ECONNRESET, ECONNREFUSED, ETIMEDOUT, ENOTFOUND, EAI_AGAIN, ENETUNREACH, EPIPE), TypeError from fetch, and common transient message patterns. These now produce an APIError with isRetryable: true so SessionRetry retries instead of surfacing a terminal error.

2. Truncate long error response bodies in ProviderError.message() — When a proxy/gateway returns an HTML error page or any long non-JSON body, e.responseBody was embedded verbatim. Now truncated at 200 chars with "..." suffix.

How did you verify your code works?

• Typecheck passes (bun typecheck in packages/opencode)
• All 33 retry classification tests pass
• All 4 account CLI tests pass
• Full repo typecheck passes (23/23 packages)

Screenshots / recordings

N/A

Checklist

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

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on the search results, I found one potentially related PR:

PR #30638 - fix(session): classify transport and timeout errors as retryable

• fix(session): classify transport and timeout errors as retryable #30638
• This PR appears to address a very similar issue: classifying transport and timeout errors as retryable rather than terminal. This overlaps with the core purpose of PR fix(opencode): retry transient network errors instead of surfacing as terminal with raw content #31440, which adds isTransientNetworkError() detection and marks those errors as retryable.

The other results (PR #29627 and #19204) are related to error handling and retries but address different specific scenarios (undici TypeError, 5xx status codes) and appear to be distinct from this PR's focus.

[github-actions]

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

[github-actions]

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