feat: add fetch_url tool for proxying HTTP GET to cluster-internal URLs

Add a new MCP tool that lets the agent fetch URLs on the cluster's
network (YARN tracking UI, Spark History Server, NodeManager web UIs)
when the agent is on a different network and cannot reach those hosts
directly.

The MCP service runs on the YARN RM node, so it can reach every host
the cluster knows about — the agent just needs a way to ask.

Security: SSRF guard via host suffix overlap
  - URL host must share >= 2 labels of suffix with the named
    Connection's yarn_rm_url host (e.g. yarn_rm_url='rm.prod.internal'
    allows 'http://nm01.prod.internal/...')
  - IP literals (10.0.0.1, ::1) rejected
  - Non-http(s) schemes (file://, gopher://, ftp://) rejected
  - Connection with no yarn_rm_url cannot use this tool
- Reuses Connection.auth_for_httpx() and verify_for_httpx() so the
  agent does not need cluster credentials
- Response body capped at 1 MB (truncated=true if larger)
- 30s timeout, follows redirects, loguru INFO audit log on every call

- spark_executor/tools/fetch_url.py: new tool + 2 helpers
  (_host_suffix_overlap, _validate_url_host)
- spark_executor/models.py: FetchUrlResult Pydantic model
- spark_executor/tools/requests.py: FetchUrlRequest with descriptions
- spark_executor/server.py: /fetch_url route, operation_id='fetch_url'
- tests/unit/test_fetch_url.py: 13 unit tests covering all guards,
  truncation, auth/SSL pass-through, redirect follow
- tests/integration/test_mcp_routes.py: assert 21 tool routes
- README.md: 1 row in Spark Executor 工具 table

Tests: 369 passed (up from 356).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -120,6 +120,7 @@ MCP 客户端需要先执行 `initialize` 握手，拿到 `mcp-session-id` 后
 | `get_external_job_status` | 查询**非本服务提交**的外部 YARN application 状态（按 `application_id` + `connection_name`） |
 | `get_external_job_result` | 查询外部 YARN application 终态结果视图 |
 | `get_external_job_logs` | 拉取外部 YARN application 的聚合日志 |
+| `fetch_url` | 代理 HTTP GET 到集群内网 URL (受 `Connection.yarn_rm_url` host 围栏约束) |
 
 ### Files MCP 工具