fix: filter empty content blocks for Bedrock provider

[JackFGreen]

Fixes Bedrock ValidationException errors caused by empty text content blocks in three scenarios:

• Empty text blocks in tool-result content arrays
• Empty text in tool output when only attachments exist
• Assistant messages with empty content string when only tool_calls present

Issue for this PR

Closes#16332

Type of change

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

What does this PR do?

• packages/opencode/src/provider/transform.ts: Added nested filtering to remove empty text blocks inside tool-result output content arrays
• packages/opencode/src/session/message-v2.ts: Skip empty text blocks when tool output has attachments; fallback to "[No output]" if content would be empty
• packages/opencode/src/provider/provider.ts: Patch assistant messages with content: "" to content: null in openai-compatible fetch hook

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How did you verify your code works?

• Ran bun test from packages/opencode - all tests passed
• Ran bun typecheck from packages/opencode - all checks passed
• Ran reproduce steps in issue several times

Screenshots / recordings

If this is a UI change, please include a screenshot or recording.

Checklist

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

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[github-actions]

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

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

PR #17363: "fix: empty tool-result content and cache control level for custom @ai…"

• fix: empty tool-result content and cache control level for custom @ai… #17363

Why it might be related: This PR also addresses empty tool-result content issues, which overlaps with the current PR's handling of empty text blocks in tool-result content arrays. Both are fixing similar validation issues related to tool-result content formatting.

The other results (PRs #16750, #16751, #8497, #8958) address related but distinct issues (tool_use/tool_result mismatches, dangling blocks, thinking blocks) rather than the specific empty content block filtering problem.