feat(yarn_client): SSL/TLS config for YARN REST connections

Add per-Connection ssl_verify / ssl_ca_bundle plus global defaults so
CDH 5 / on-prem clusters with self-signed certs or custom CA bundles can
be queried without patching code.

- Connection gets ssl_verify (bool|None) and ssl_ca_bundle (str|None)
- Settings gets ssl_verify_default and ssl_ca_bundle_default
- New YarnClientConfig dataclass carries the resolved verify= value
- _request passes verify= through to httpx.request
- All public yarn_client functions now take YarnClientConfig instead of
  a bare yarn_rm_url string; tool call sites resolve the Connection
- SaveConnectionRequest exposes the two new fields

Tests cover per-connection CA bundle, per-connection verify=False,
global default fallback, and connection-not-found error.

spark_executor/tools/kill.py

@@ -5,7 +5,8 @@
 """
 from common.logging import logger
 from spark_executor.core.job_store import JobStore
-from spark_executor.core.yarn_client import kill_application
+from spark_executor.core.yarn_client import YarnClientConfig, kill_application
+from spark_executor.tools.connections import store as conn_store
 
 store = JobStore()
 
@@ -15,7 +16,11 @@ def kill_job(job_id: str) -> dict[str, str]:
     job = store.get(job_id)
     if job is None:
         raise KeyError(f"Unknown job_id: {job_id}")
-    kill_application(job.application_id, job.yarn_rm_url)
+    conn = conn_store.get(job.connection)
+    if conn is None:
+        raise KeyError(f"Connection not found: {job.connection}")
+    config = YarnClientConfig.from_connection(conn)
+    kill_application(job.application_id, config)
     logger.info(f"kill_job ok job_id={job_id} application_id={job.application_id}")
     return {
         "job_id": job_id,