[BUG] API Error 400 after v1 update (f0c8eb2) - PR review workflows fail with "tool use concurrency issues"

## Summary

PR review workflows started failing with `API Error: 400 due to tool use concurrency issues` after `claude-code-action@v1` auto-updated from SHA `6337623` (Dec 2, 2025) to `f0c8eb2` (Dec 9, 2025, Claude Code 2.0.62).

## Environment

- **Action version**: `anthropics/claude-code-action@v1` (auto-resolving to `f0c8eb2`)
- **Claude Code version**: 2.0.62
- **Workflow type**: PR review with pending review creation
- **Provider**: Anthropic API (1p)
- **GitHub Runner**: ubuntu-latest

## Error Details

```json
{
"type": "result",
"subtype": "success",
"is_error": true,
"duration_ms": 10084,
"num_turns": 2,
"result": "API Error: 400 due to tool use concurrency issues.",
"error": "invalid_request"
}
```

## Configuration

```yaml
- name: Claude Code Review
uses: anthropics/claude-code-action@v1
with:
anthropic_api_key: ${{ secrets.ANTHROPIC_API_KEY }}
show_full_output: true
claude_args: |
--max-turns 30
--model claude-sonnet-4-5-20250929
--allowedTools "mcp__github__create_pending_pull_request_review,mcp__github__get_pull_request_diff,mcp__github__add_comment_to_pending_review,mcp__github__submit_pending_pull_request_review"
```

## Reproduction Steps

1. Use `claude-code-action@v1` (defaults to latest f0c8eb2)
2. Configure PR review workflow with `allowedTools` parameter
3. Open or reopen a pull request to trigger the workflow
4. Observe failure after 2-3 turns with "API Error: 400 due to tool use concurrency issues"

## Expected Behavior

The workflow should complete successfully and post PR review comments, as it did with SHA `6337623`.

## Actual Behavior

- Workflow fails consistently after 2-3 turns
- Error message: "API Error: 400 due to tool use concurrency issues"
- No PR review is posted
- Tool execution appears to succeed (tool_use/tool_result pairs are properly matched)

## Analysis

### Last Working Version
- **SHA**: `6337623ebba10cf8c8214b507993f8062fd4ccfb` (Dec 2, 2025)
- **Status**: ✅ Consistently successful
- **Test runs**: Multiple PRs reviewed successfully (Dec 5-9, 2025)

### Breaking Version
- **SHA**: `f0c8eb29807907de7f5412d04afceb5e24817127` (Dec 9, 2025)
- **Changes**: Claude Code version bump 2.0.61 → 2.0.62
- **Status**: ❌ Consistently fails with 400 error

### Changes Between Versions

Between these commits, the following PRs were merged:

1. **#719**: "fix: accumulate multiple --allowedTools flags" ⚠️ **Suspected cause**
- Modified how allowedTools/disallowedTools are processed
- May have introduced regression in tool parameter handling

2. **#710**: "fix: prevent TOCTOU race condition on issue/PR body edits"
- Security fix for body content validation

3. **Other PRs**: Various minor fixes

### Suspicious Finding

Tool execution logs show:
- Tool calls are properly formed with valid IDs
- Tool results are returned successfully
- tool_use/tool_result pairs are correctly matched
- **However**, Anthropic API still returns 400 error

This suggests the issue may be in how the tool list or parameters are serialized to the API request, possibly related to #719's changes in `allowedTools` accumulation logic.

## Workaround

Pin to the last working version:

```yaml
uses: anthropics/claude-code-action@6337623ebba10cf8c8214b507993f8062fd4ccfb
```

This resolves the issue and allows PR reviews to complete successfully.

## Impact

- **Severity**: P1 - All users using `@v1` tag are affected
- **Scope**: PR review workflows fail completely
- **Workaround exists**: Yes (version pinning)

## Additional Context

- Similar error message appeared in anthropics/claude-code#8763, but that was related to actual tool concurrency
- In this case, tools are executed sequentially (only 1 tool call per turn observed)
- Error persists even with explicit sequential execution constraints in prompt

## Logs

Detailed error log (with show_full_output: true)

```
pr-review Claude Code Review {"type":"result","subtype":"success","is_error":true,"duration_ms":10084,"duration_api_ms":32359,"num_turns":2,"result":"API Error: 400 due to tool use concurrency issues.","session_id":"697908da-f203-4ce3-9758-e24f7513e039","total_cost_usd":0.21781100000000003,"usage":{"input_tokens":9,"cache_creation_input_tokens":46642,"cache_read_input_tokens":2320,"output_tokens":316}}
```

Tool execution sequence:
1. ✅ `mcp__github__get_pull_request` (toolu_015BVVwKWnHWfpKay5gbQBe2) - Success
2. ❌ Anthropic API returns 400 error

## Request

Please investigate PR #719's changes to `allowedTools` accumulation logic, as this appears to be the most likely cause of the regression.