feat: add list_applications tool for YARN application enumeration

Add a new MCP tool that queries YARN's /ws/v1/cluster/apps endpoint
through a named Connection, returning a list of ApplicationSummary
records. Bypasses the local JobStore — useful for enumerating apps
that were not submitted through this service.

API:
  list_applications(
    connection_name: str,           # required, which YARN cluster
    state: str | None = None,       # YARN state filter: NEW/NEW_SAVING/
                                    # SUBMITTED/ACCEPTED/RUNNING/
                                    # FINISHED/FAILED/KILLED
    queue: str | None = None,       # YARN queue filter
    limit: int = 100,               # cap on returned apps (YARN has no
                                    # offset-based pagination; combine
                                    # state/queue filters for big clusters)
  ) -> list[ApplicationSummary]

Implementation:
  - yarn_client.list_applications(config, *, state, queue, limit) -> list[dict]
    Returns raw YARN app dicts; raises YarnError on 4xx/5xx; returns
    [] on 404 (no apps match). Uses the existing _request helper,
    which now accepts a "params" kwarg for query strings (one-line
    additive change).
  - external_jobs.list_applications(connection_name, state, queue, limit)
    -> list[ApplicationSummary]. Looks up the Connection, builds the
    YarnClientConfig, calls the yarn_client function, maps each raw
    YARN dict to ApplicationSummary (mirroring the manual field-mapping
    style of get_job_result). The yarn_client function is imported
    as "list_applications_yarn" to avoid name collision.
  - ApplicationSummary: 12-field Pydantic model with snake_case names
    (application_id, name, user, queue, state, final_status,
    application_type, application_tags, started_time, finished_time,
    tracking_url, progress). Unused YARN fields (memorySeconds,
    vcoreSeconds, preemptedResource*, etc.) are not exposed.
  - ListApplicationsRequest: Pydantic body model with Field(description=)
    for LLM-facing schema.
  - /list_applications route registered with operation_id=
    "list_applications", placed next to the other external YARN tools.

Tests:
  - 8 new unit tests in test_external_jobs.py (happy path, state/queue/
    limit pass-through, default limit, empty list, missing connection,
    full field mapping).
  - test_mcp_routes.py: assert 23 tool routes.
  - README: list_applications row added to the Spark Executor table.

Tests: 390 passed (was 382, +8 net).

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

README.md

@@ -121,6 +121,7 @@ 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 |
+| `list_applications` | 列出 YARN 上所有应用（按 `state` / `queue` / `limit` 过滤），绕过 JobStore |
 | `fetch_url` | 代理 HTTP GET 到集群内网 URL (host 受 `Connection.url_allowlist` glob allowlist 约束, 空则全拒) |
 
 ### Files MCP 工具