fix: filter empty/whitespace text content blocks at provider boundaries

[RomneyDa]

Summary

Skip empty and whitespace-only text content blocks at each provider's message conversion layer, preventing 400 errors from Anthropic and Bedrock.

3 targeted changes, +14/-5 lines, zero upstream behavioral changes.

Changes

File	Change	Issues fixed
core/llm/llms/Anthropic.ts	.trim() check in convertMessageContentToBlocks	#11446
core/llm/llms/Bedrock.ts	Skip empty text in _convertMessageContentToBlocks	#9765, #9767, #11264, #11497
packages/openai-adapters/src/apis/Bedrock.ts	Skip empty text in user message conversion	#10148, #10804, #11045

What this does NOT change

• No upstream message filtering or compilation logic
• No changes to messages.ts, countTokens.ts, or toResponsesInput
• Existing " " fallbacks for string content are untouched
• No changes to the OpenAI adapter's toChatMessage conversion

Issues fixed

Fixes #9765, fixes #9767, fixes #10148, fixes #10804, fixes #11045, fixes #11264, fixes #11446, fixes #11497

Test plan

• openaiTypeConverters.test.ts — 24/24 pass
• Prettier clean
• Manual: Bedrock ConverseStream
• Manual: Bedrock InvokeModelWithResponseStream
• Manual: Anthropic direct

[RomneyDa]

Why targeted provider-level fixes instead of upstream changes

We initially explored a broader approach (see #11808) that modified the shared message filtering/compilation layer (messages.ts, countTokens.ts) — replacing addSpaceToAnyEmptyMessages with stripEmptyContentParts, fixing chatMessageIsEmpty to handle array content, and changing fallback behavior in toResponsesInput.

We opted against that because:

• Changing content: null or content: "" for assistant messages could break OpenAI-compatible providers (LM Studio, Ollama, vLLM, etc.) that expect non-null content
• Removing the " " fallback in addSpaceToAnyEmptyMessages changes behavior for every provider, not just the ones with reported bugs
• Modifying toResponsesInput to drop empty assistant messages could break Responses API ID chains
• Silencing the Bedrock empty-turn error would mask bugs instead of surfacing them

Instead, this PR adds .trim() guards at the 4 exact locations where text content blocks are constructed for providers that reject empty/whitespace text. Each fix is a one-liner that only affects the provider with the reported bug.

[continue]

Documentation Review

No documentation updates are needed for this PR.

Reasoning:

• This is an internal bug fix to provider message conversion logic that filters empty/whitespace text content blocks before they reach provider APIs
• The changes are entirely within internal implementation details (convertMessageContentToBlocks, _convertMessageContentToBlocks, toChatMessage) that users never interact with
• No new user-facing features, configuration options, or behavior changes are introduced
• The fix resolves 400 errors that were implementation artifacts, not something users could troubleshoot via configuration

The existing documentation for Anthropic and Bedrock providers remains accurate and complete.

[continue]

Test Coverage Review

This PR introduces a bug fix to filter out empty/whitespace-only text content blocks across multiple provider implementations. Per testing guidelines, bug fixes should include regression tests.

Missing Test Coverage

1. core/llm/openaiTypeConverters.test.ts

The toChatMessage function was modified but has no direct test coverage. Consider adding tests for:

• Assistant message with content: [{type: 'text', text: ' '}] (whitespace only) should filter out the text part
• Assistant message with content: [{type: 'text', text: ''}] (empty) should filter out the text part
• Mixed content with empty/whitespace text parts alongside valid parts should only keep valid parts

2. core/llm/llms/Anthropic.vitest.ts

The content block conversion was modified. Consider adding tests for:

• Empty string content produces no text blocks
• Whitespace-only string content produces no text blocks
• Array content with whitespace-only text parts filters them out

3. core/llm/llms/Bedrock.test.ts

The content conversion was modified. Consider adding to the message conversion describe block:

• Empty string content produces empty content array
• Whitespace-only string content produces empty content array
• Array content with whitespace-only text parts filters them out

4. packages/openai-adapters/src/apis/Bedrock.ts

No test file exists for this module. Consider adding tests for the message conversion helper.

Why These Tests Matter

The bug being fixed (providers rejecting empty/whitespace text blocks) could regress if:

• Future refactors accidentally remove the .trim() checks
• Similar functions are added without the same safeguards

Simple unit tests for each affected function would catch these regressions.

[cubic-dev-ai]

No issues found across 4 files