feat(result): get_job_result tool for terminal view of Spark job

`get_job_status` returns YARN state + the raw response blob, so the
terminal fields (finalStatus, diagnostics, trackingUrl, startedTime,
finishedTime) are buried inside `raw` and not surfaced in a structured
form. Add a new tool that parses them.

`get_job_result(job_id)` reuses `yarn_client.get_application_status` and
extracts:
  - finalStatus  (SUCCEEDED / FAILED / KILLED / UNDEFINED)
  - diagnostics  (YARN final message)
  - tracking_url (Spark Web UI)
  - started_time / finished_time (epoch ms)

All five fields are optional: running jobs have no `finishedTime`, and
older YARN versions (CDH 5 / H2) may omit some fields. Missing fields
stay None — never raise.

Coexistence with `get_job_status` is intentional: the latter is for
polling the running YARN state, the former is the terminal view.

Tests: 4 cases — happy path, running job (no finishedTime), bare-minimum
raw (all optionals None), unknown job_id raises KeyError. uv run pytest
-> 171 passed.

spark_executor/server.py

@@ -37,6 +37,8 @@ from spark_executor.tools.submit import (
     prepare_submit_job,
 )
 
+from spark_executor.tools.result import get_job_result
+
 app = FastAPI(title="Spark Executor MCP", version="0.0.1", description="Spark Executor MCP Server")
 
 
@@ -149,6 +151,20 @@ def _get_job_status(req: JobIdRequest):
     return get_job_status(req.job_id)
 
 
+@app.post(
+    "/get_job_result",
+    summary="Query YARN for a job's terminal result view",
+    description=(
+        "Return a terminal-oriented view of a Spark job: final_status, "
+        "diagnostics, tracking_url, started_time, and finished_time. "
+        "This is distinct from get_job_status, which is for polling the "
+        "running YARN state and returns the raw YARN response."
+    ),
+)
+def _get_job_result(req: JobIdRequest):
+    return get_job_result(req.job_id)
+
+
 @app.post(
     "/get_job_logs",
     summary="Fetch aggregated container logs for a job",