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/models.py

@@ -24,6 +24,16 @@ class JobStatus(BaseModel):
     raw: str = Field(default="")
 
 
+class JobResult(BaseModel):
+    application_id: str
+    state: str
+    final_status: str | None = None
+    diagnostics: str | None = None
+    tracking_url: str | None = None
+    started_time: int | None = None
+    finished_time: int | None = None
+
+
 class SubmitResult(BaseModel):
     job_id: str
     application_id: str