fix: filter empty content blocks for Bedrock provider

[Tom-Ryder]

Issue for this PR

Closes #11210

Type of change

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

What does this PR do?

normalizeMessages() filters empty text/reasoning parts for @ai-sdk/anthropic but not @ai-sdk/amazon-bedrock. The Bedrock Converse API has the same constraint — empty content arrays cause "content field is empty" and invalid cache point errors that permanently break the session. Extended the existing guard to also cover Bedrock.

Note: this fixes the empty content filtering side of #11210. The separate Bedrock ConverseAPI tool call issues flagged in the thread will need a follow-up.

How did you verify your code works?

• bun turbo typecheck passes
• Added 6 tests mirroring the existing anthropic empty content filtering suite, all pass
• Existing tests unchanged and passing

Screenshots / recordings

N/A — backend logic change.

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 my search, I found related PRs but no direct duplicates of PR #14586:

Related PRs:

1. PR fix(provider): trim whitespace and filter empty content in transformations #7086 - "fix(provider): trim whitespace and filter empty content in transformations"

   • Addresses similar empty content filtering but appears to be from an earlier iteration

2. PR fix: expand Anthropic detection and strip whitespace-only text blocks #12634 - "fix: expand Anthropic detection and strip whitespace-only text blocks"

   • Related to the anthropic filtering mentioned in your PR description

3. PR fix: strip incompatible thinking blocks when switching to Claude (improved) #11882 - "fix: strip incompatible thinking blocks when switching to Claude (improved)"

   • Part of the broader content filtering effort for different providers

These are related to content filtering improvements, but none appear to be addressing the exact same issue (filtering empty content blocks for Bedrock specifically with the normalizeMessages() function).

No duplicate PRs found

[github-actions]

Thanks for your contribution!

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