fix: add final empty-content guard in message() pipeline

[Oxygen56]

Closes #23260
Closes #26320

Type of change

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

What does this PR do?

Adds a provider-agnostic empty-content guard at the end of the message() function in packages/opencode/src/provider/transform.ts. The guard filters out messages with empty content (empty string, empty array, or all-empty text/reasoning parts) before they reach any provider API.

Previously only Anthropic and Bedrock had provider-specific guards; every other provider — including DeepSeek — could receive messages with empty content and fail with hard errors that permanently break the active session (e.g. DeepSeek returns 400 Bad Request: messages.N: all messages must have non-empty content).

The fix is a single O(n) filter (~15 lines) placed at the end of message(), provider-agnostic by design, catching all current and future empty-content sources (unsupportedParts, normalizeMessages, interleaved reasoning filter, etc.).

How did you verify your code works?

• Added 5 unit tests in packages/opencode/test/provider/transform.test.ts covering: empty string content, empty array content, all-empty text parts, mixed valid/invalid messages, and provider-agnostic behavior (verified with both DeepSeek and OpenAI models)
• The DeepSeek reproduction case (messages.N: all messages must have non-empty content) is documented in bug: message pipeline has no final empty-content guard, causing ValidationException on Bedrock and hard failures on other providers #23260 (comment)

Screenshots / recordings

N/A (not a UI change)

Checklist

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

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• PR description is missing required template sections. Please use the PR template.

Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

If you believe this was flagged incorrectly, please let a maintainer know.

[github-actions]

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

Based on my search, I found several potentially related PRs that address similar issues with empty content or message transformation:

Potentially Related PRs:

1. fix(session): skip empty text parts in assistant message replay #31045 - fix(session): skip empty text parts in assistant message replay

   • Handles empty text parts in messages, similar concept but in message replay context

2. fix(provider): drop whitespace-only text blocks for Anthropic/Bedrock #31376 - fix(provider): drop whitespace-only text blocks for Anthropic/Bedrock

   • Addresses empty/whitespace content filtering at the provider level, same issue area

3. refactor(provider): extract helpers from normalizeMessages #31599 - refactor(provider): extract helpers from normalizeMessages

   • Refactoring in the same message normalization pipeline that this PR modifies

4. fix: preserve thinking/redacted_thinking blocks in Anthropic message transforms #23755 - fix(provider): preserve thinking/redacted_thinking blocks in Anthropic message transforms

   • Related to message transformation logic and provider-specific handling

These PRs are in the same message() transformation pipeline area, but none appear to be direct duplicates. The current PR #33647 appears to add a comprehensive final guard that complements these existing fixes.

[github-actions]

This pull request has been automatically closed because it was not updated to meet our contributing guidelines within the 2-hour window.

Feel free to open a new pull request that follows our guidelines.