SDK AJV crash after successful multi-turn execution with --agent flag

## Description

When using `--agent` flag to load a custom agent definition, the SDK crashes with an AJV validation error **after** successfully completing multiple turns and spending API credits. This is different from issues #852/#892 which crash on initialization.

## Environment

- **Action version:** Tested both `@v1` (SHA:6867bb3ab0b2c0a10629b6823e457347e74ad6d2) and pinned `@01e756b` (SDK 0.2.25)
- **Agent SDK versions:** Both 0.2.31 and 0.2.25 exhibit the issue
- **Custom agent:** Using `--agent pr-review` with a custom `.claude/agents/pr-review.md` definition

## Key Observations

- **Work completes successfully:** 4 turns, $0.27 cost, 17 seconds
- **Crash happens post-execution:** After the result JSON is emitted
- **Comments are partially posted:** The review work succeeds before the crash

## Error Output

```json
{
"type": "result",
"subtype": "success",
"is_error": true,
"duration_ms": 16752,
"num_turns": 4,
"total_cost_usd": 0.27037100000000003,
"permission_denials": []
}
SDK execution error: 14 | depsCount: ${Q},
15 | deps: ${$}}`};var Pj={keyword:"dependencies"...

error: Claude Code process exited with code 1
```

## Workflow Configuration

```yaml
- name: Run PR Review
uses: anthropics/claude-code-action@01e756b34ef7a1447e9508f674143b07d20c2631
with:
anthropic_api_key: ${{ secrets.ANTHROPIC_API_KEY }}
claude_args: |
--agent pr-review
--allowedTools "Task,Read,Grep,Glob,mcp__github_inline_comment__create_inline_comment,Bash(git diff:*),Bash(git log:*),Bash(gh pr comment:*),Bash(gh api:*)"
prompt: Review PR #${{ github.event.pull_request.number }}.
```

## Difference from #852/#892

| Issue #852/#892 | This Issue |
|-----------------|------------|
| Crashes on turn 1, $0 cost | Crashes after 4+ turns, $0.27 spent |
| SDK initialization failure | Post-execution validation failure |
| No API calls made | Work completes successfully |
| Pinning to SDK 0.2.25 fixes it | Pinning does NOT fix it |

## Hypothesis

The crash appears to happen during output serialization/validation when the agent response contains certain structures. The custom agent may produce output that triggers a bug in AJV's schema validation when the SDK tries to process the result.

## Workaround Attempts

- ❌ Pinned to SDK 0.2.25 (`@01e756b`) - same crash
- ❌ Different allowedTools combinations - same crash

## Repository

https://github.com/inkeep/agents (private, can provide access if needed)

Failed runs: https://github.com/inkeep/agents/actions/runs/21688290864