fix(api): map CircuitBreakerOpen to UNAVAILABLE with retry_after

[akushonkamen]

Description

本 PR 修复 embedding 熔断器打开（CircuitBreakerOpen）经 search / find 路由外泄时被映射成不透明 500 INTERNAL 的问题：客户端现在会收到可重试的 503 UNAVAILABLE，并携带 retry_after 以便退避。

Human Involvement

• A human participated in the implementation or review loop
• This PR was generated entirely by AI agents without human participation in the loop

AI 披露：本修复由 AI agent 定位根因、实现并验证；认领评论（含与 #6045 的划界）同样由 AI 生成。

Related Issue

Fixes #6139

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

根因：map_exception（openviking/server/error_mapping.py:477 起）没有 CircuitBreakerOpen 的 isinstance 分支，且熔断消息（如 "Circuit breaker is OPEN, retry after 13s"）不含 upstream marker / HTTP 状态码 / 兜底子串可命中，最终走到函数末尾的 fallback return None（error_mapping.py:617）。app.py 的 general_error_handler（openviking/server/app.py:565）对 None 落到默认 500 INTERNAL。CircuitBreakerOpen 唯一携带 HTTP 语义的外溢路径是 embedding 资源 _execute 打上 retry_after 后 re-raise（openviking/config/embedding.py:243-245）；另两处捕获点（openviking/storage/collection_schemas.py:755、openviking/storage/queuefs/semantic_processor.py:546）均为内部 requeue，不外泄到 API 层。

修法：map_exception 在 GitConcurrentCommitError 分支之后新增 isinstance(exc, CircuitBreakerOpen) 分支（error_mapping.py:509-516），镜像既有 AGFS UnavailableError 分支风格：构造 UnavailableError("embedding provider", reason=str(exc))（code=UNAVAILABLE → HTTP 503），并把 embedding 路径标注的 retry_after 补进 details，供客户端退避。

测试：tests/server/test_error_mapping.py 新增 4 个回归测试——熔断带 / 不带 retry_after、文本 429 → RESOURCE_EXHAUSTED、litellm APIConnectionError → UNAVAILABLE（后两者钉住 #6045 已在 main 修好的路径，防回归）。

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

本地验证（克隆自 origin/main = 84cc2b4，与工单 triage 基线一致；uv sync --frozen --extra test 后全部实跑）：

1. 红态探针（改前）：uv run --no-sync python -c "map_exception(CircuitBreakerOpen('Circuit breaker is OPEN, retry after 13s'))" → None（复现 500 根因）。
2. 基线探针：文本 'Error code: 429' → RESOURCE_EXHAUSTED；litellm APIConnectionError(status_code=500) → UNAVAILABLE，均已在 main 绿（与 fix(api): 修正输入错误与模型失败的 HTTP 状态码 #6045 triage 一致）。
3. 测试级红：stash 修复后 uv run --no-sync python -m pytest tests/server/test_error_mapping.py -q -k "circuit or litellm or text_429" → 2 failed 2 passed（熔断 2 例红、429/litellm 2 例绿）。
4. 修复后同命令 → 4 passed。
5. 全文件：timeout 300 uv run --no-sync python -m pytest tests/server/test_error_mapping.py -q → 40 passed, 8 warnings（= 36 既有 + 4 新增）。
6. lint：uv run --no-sync ruff check openviking/server/error_mapping.py tests/server/test_error_mapping.py → All checks passed。ruff format --check 报 1 处 reformat，经 git show HEAD:tests/... 对纯净文件复验为 HEAD 既有 drift（test_git_auth_failure_remains_a_client_error 尾段），本 PR 新增行零 format diff，未顺带重排（保持最小 diff）。

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

• Commit 带 DCO：Signed-off-by: akushonkamen <akushonkamen@163.com>，Conventional Commits 格式 fix(api):。
• 查重：issue timeline 中 akushonkamen 0 个关联事件、gh search prs 6139 全状态为空、issue comments 无重复实现。

[akushonkamen]

Hi — a quick note on the failing CI check, since everything else is green:

API & CLI Integration Tests (ubuntu-24.04) failed for environmental reasons unrelated to this PR. All 5 failed tests (filesystem/test_fs_cp.py, test_fs_read_write.py, …) fail with the same external-service error:

PROCESSING_ERROR: Failed to generate embedding: Volcengine embedding failed:
Error code: 401 - AuthenticationError: The API key format is incorrect

i.e. the CI integration environment's Volcengine credentials are currently rejected — this is not touched by this PR:

• Same failure on unrelated branches: the identical 5 tests fail with the identical 401 signature on other PR branches today (e.g. run 38754191585 on fix/6129-embedding-token-estimate), and this workflow shows failures across 5+ unrelated PR branches on 2026-10-15 — while the main run at 13:07 UTC succeeded, suggesting PR-branch runs are hitting an invalid/missing API key.
• Out of scope of the diff: this PR only changes openviking/server/error_mapping.py (maps CircuitBreakerOpen → UNAVAILABLE with retry_after) and its unit tests in tests/server/test_error_mapping.py — no filesystem or embedding code paths.
• Local evidence (all green): python -m pytest tests/server/test_error_mapping.py → 40 passed (includes the 4 new tests added by this PR); the other PR-triggered checks on this PR (check-deps, plugin-tests, plugin-versions) all passed.

Happy to re-run once the CI credentials are fixed, and of course glad to address any review feedback on the change itself.