refactor: replace yarn CLI shell-out with YARN REST API

yarn_client.py no longer invokes the 'yarn' binary via subprocess; it uses
httpx against /ws/v1/cluster/apps/* endpoints. This means the runtime image
no longer needs the Hadoop client installation — the only YARN-side
dependency left in the container is the config dir consumed by
spark-submit itself.

New model field:
  - Job.yarn_rm_url: str | None
  - PendingSubmission.yarn_rm_url: str | None (snapshotted at prepare)

prepare_submit_job snapshots Connection.yarn_rm_url into the pending
record (consistent with the existing master/deploy_mode/spark_conf
snapshot pattern); confirm_submit_job copies it onto the Job so
status/logs/kill can use it without re-looking-up the connection.

Resolution order for the RM URL at runtime:
  1. Job.yarn_rm_url (preferred — survives connection edits/deletes)
  2. Connection.yarn_rm_url fallback (if a future tool is added that
     doesn't go through a Job)
  3. YARN_RESOURCE_MANAGER_URL env var

Errors:
  - YarnConfigError (HTTP 4xx semantics) when URL is missing/malformed
  - YarnError for HTTP 4xx/5xx from the RM, network failures, missing
    state field, or unparseable log responses

10 new tests in test_yarn_client.py cover the REST surface:
success, 404, 5xx, missing state field, env-var fallback, malformed
URL, log 404 with log-aggregation hint, kill PUT body shape, and
httpx connection-error wrapping.

tests/unit/test_models.py

@@ -11,11 +11,25 @@ def test_job_roundtrip():
         queue="default",
         submit_time=datetime(2026, 6, 24, 10, 0, 0),
         connection="prod-yarn",
+        yarn_rm_url="http://rm:8088",
     )
     dumped = job.model_dump()
     assert dumped["job_id"] == "abc123"
     assert dumped["application_id"] == "application_17400000001"
     assert dumped["connection"] == "prod-yarn"
+    assert dumped["yarn_rm_url"] == "http://rm:8088"
+
+
+def test_job_yarn_rm_url_optional():
+    job = Job(
+        job_id="j1",
+        application_id="application_1",
+        script_path="/tmp/x.py",
+        queue="default",
+        submit_time=datetime(2026, 6, 24),
+        connection="dev",
+    )
+    assert job.yarn_rm_url is None
 
 
 def test_job_status_default_raw():
@@ -63,6 +77,26 @@ def test_pending_submission_defaults_to_pending_status():
     assert p.error is None
     assert p.job_id is None
     assert p.application_id is None
+    assert p.yarn_rm_url is None  # default for connections without one set
+
+
+def test_pending_submission_carries_yarn_rm_url_snapshot():
+    """snapshotting yarn_rm_url lets prepare/confirm survive connection edits."""
+    p = PendingSubmission(
+        pending_id="p_x",
+        connection="prod",
+        master="yarn",
+        deploy_mode="cluster",
+        yarn_rm_url="http://rm-prod:8088",
+        script_path="/tmp/j.py",
+        queue="default",
+        executor_memory="4G",
+        executor_cores=2,
+        num_executors=2,
+        spark_conf={},
+        created_at=datetime(2026, 6, 24),
+    )
+    assert p.yarn_rm_url == "http://rm-prod:8088"
 
 
 def test_pending_submission_can_record_outcome():