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>

spark_executor/server.py

@@ -21,6 +21,7 @@ from spark_executor.tools.external_jobs import (
     get_external_job_logs,
     get_external_job_status,
     get_external_job_result,
+    list_applications,
 )
 from spark_executor.tools.fetch_url import fetch_url
 from spark_executor.tools.requests import (
@@ -30,6 +31,7 @@ from spark_executor.tools.requests import (
     ExternalJobLogsRequest,
     ExternalJobStatusRequest,
     ExternalJobResultRequest,
+    ListApplicationsRequest,
     FetchUrlRequest,
     GetJobLogsRequest,
     JobIdRequest,
@@ -346,6 +348,34 @@ def _get_external_job_result(req: ExternalJobResultRequest):
     return get_external_job_result(req.application_id, req.connection_name)
 
 
+@app.post(
+    "/list_applications",
+    operation_id="list_applications",
+    summary="List YARN applications on a cluster, optionally filtered",
+    description=(
+        "Query YARN's /ws/v1/cluster/apps endpoint through the named "
+        "Connection, returning a list of ApplicationSummary records. "
+        "Bypasses the local JobStore — useful for enumerating apps that "
+        "were not submitted through this service.\n\n"
+        "**Filters:** state (YARN state, e.g. 'RUNNING', 'FINISHED', "
+        "'FAILED'), queue (YARN queue name), limit (default 100, max ~10000). "
+        "YARN has no offset-based pagination, so for large clusters combine "
+        "state/queue filters to scope the result. The `FINISHED` state "
+        "covers SUCCEEDED/FAILED/KILLED.\n\n"
+        "Returns an empty list if no apps match. The Connection's auth_type "
+        "/ auth_user / auth_password / ssl_verify / ssl_ca_bundle are reused "
+        "for the request."
+    ),
+)
+def _list_applications(req: ListApplicationsRequest):
+    return list_applications(
+        req.connection_name,
+        state=req.state,
+        queue=req.queue,
+        limit=req.limit,
+    )
+
+
 # --- Connection management tools ---
 
 @app.post(