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

[Oxygen56]

Issue for this PR

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 (e.g. DeepSeek: 400 Bad Request: messages.N: all messages must have non-empty content).

The fix is a single O(n) filter (~15 lines) at the end of message(), provider-agnostic by design.

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 model fixtures).

The DeepSeek reproduction case is documented in #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

Re-creation of #33647 and #33706, both closed by bot timer rather than content issues.

[github-actions]

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

Based on my search results, I found several related PRs that address similar issues with empty content handling in messages:

Related PRs:

1. fix(provider): drop whitespace-only text blocks for Anthropic/Bedrock #31376 - fix(provider): drop whitespace-only text blocks for Anthropic/Bedrock - Similar provider-specific empty content filtering
2. 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
3. refactor(provider): extract helpers from normalizeMessages #31599 - refactor(provider): extract helpers from normalizeMessages - Refactors message normalization logic
4. fix: preserve thinking/redacted_thinking blocks in Anthropic message transforms #23755 - fix: preserve thinking/redacted_thinking blocks in Anthropic message transforms - Related to Anthropic message handling

These are contextually related but appear to address different aspects of message handling. The most relevant is #31376 which also filters empty/whitespace content, though it was provider-specific to Anthropic/Bedrock while PR #34272 extends this to be provider-agnostic.

No direct duplicate PRs found - PR #34272 appears to be a legitimate new approach to consolidate empty-content filtering across all providers at the pipeline level rather than provider-specific implementations.

[github-actions]

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