feat: structured DEBUG/INFO logging via loguru

- common/logging.py: improve format to timestamp|LEVEL|module:func:line - message
- core/ layer: DEBUG log every subprocess invocation (cmd, rc, byte counts),
  JSON load/dump events, parsed application_id. ERROR log on failures.
- tools/ layer: DEBUG log every public tool entry with key parameters,
  INFO log on business outcomes (saved/submitted/killed/...).
- New tests/unit/test_logging.py: capture loguru output via in-memory sink
  and assert DEBUG + INFO messages are emitted for representative flows.

spark_executor/core/yarn_client.py

@@ -6,6 +6,8 @@
 import re
 import subprocess
 
+from common.logging import logger
+
 
 class YarnError(Exception):
     """Raised when a yarn CLI invocation fails."""
@@ -15,33 +17,54 @@ _STATE_RE = re.compile(r"State\s*:\s*(\S+)")
 
 
 def _run(cmd: list[str]) -> "subprocess.CompletedProcess[str]":
-    return subprocess.run(cmd, capture_output=True, text=True, errors="replace")
+    logger.debug(f"yarn _run exec: {cmd}")
+    result = subprocess.run(cmd, capture_output=True, text=True, errors="replace")
+    logger.debug(
+        f"yarn _run done rc={result.returncode} "
+        f"stdout_len={len(result.stdout)} stderr_len={len(result.stderr)}"
+    )
+    return result
 
 
 def get_application_status(application_id: str) -> tuple[str, str]:
     proc = _run(["yarn", "application", "-status", application_id])
     if proc.returncode != 0:
+        logger.error(
+            f"yarn application -status failed (rc={proc.returncode}) for "
+            f"{application_id}: {proc.stderr[:500]}"
+        )
         raise YarnError(
             f"yarn application -status failed (rc={proc.returncode}): {proc.stderr}"
         )
     match = _STATE_RE.search(proc.stdout)
     if not match:
         raise YarnError(f"Could not parse YARN state from output: {proc.stdout!r}")
-    return match.group(1), proc.stdout
+    state = match.group(1)
+    logger.info(f"yarn status {application_id} -> {state}")
+    return state, proc.stdout
 
 
 def get_application_logs(application_id: str) -> str:
     proc = _run(["yarn", "logs", "-applicationId", application_id])
     if proc.returncode != 0:
+        logger.error(
+            f"yarn logs failed (rc={proc.returncode}) for {application_id}: {proc.stderr[:500]}"
+        )
         raise YarnError(
             f"yarn logs failed (rc={proc.returncode}): {proc.stderr}"
         )
+    logger.info(f"yarn logs {application_id} -> {len(proc.stdout)} chars")
     return proc.stdout
 
 
 def kill_application(application_id: str) -> None:
     proc = _run(["yarn", "application", "-kill", application_id])
     if proc.returncode != 0:
+        logger.error(
+            f"yarn application -kill failed (rc={proc.returncode}) for "
+            f"{application_id}: {proc.stderr[:500]}"
+        )
         raise YarnError(
             f"yarn application -kill failed (rc={proc.returncode}): {proc.stderr}"
         )
+    logger.info(f"yarn kill {application_id} -> ok")