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.

tests/unit/test_logging.py

@@ -0,0 +1,63 @@
+# coding=utf-8
+import io
+from pathlib import Path
+
+import pytest
+
+from spark_executor.core import connection_store, pending_store
+from spark_executor.core.connection_store import ConnectionStore
+from spark_executor.core.pending_store import PendingStore
+from spark_executor.models import Connection
+from spark_executor.tools import connections, submit
+
+
+@pytest.fixture(autouse=True)
+def _fresh_stores(tmp_path: Path, monkeypatch):
+    monkeypatch.setattr(connection_store, "DEFAULT_DATA_DIR", str(tmp_path))
+    monkeypatch.setattr(connection_store, "store", ConnectionStore())
+    monkeypatch.setattr(pending_store, "DEFAULT_DATA_DIR", str(tmp_path))
+    monkeypatch.setattr(pending_store, "store", PendingStore())
+    connections.store = connection_store.store
+    submit.conn_store = connection_store.store
+    submit.pending_store = pending_store.store
+    submit.job_store = submit.job_store.__class__()  # fresh in-memory job store
+
+
+@pytest.fixture
+def log_capture():
+    """Attach an in-memory sink to loguru so tests can assert on emitted lines."""
+    from common.logging import logger
+    buf = io.StringIO()
+    handler_id = logger.add(buf, level="DEBUG", format="{level}|{message}")
+    yield buf
+    logger.remove(handler_id)
+
+
+def test_save_connection_emits_info_log(log_capture):
+    connections.save_connection(name="prod", master="yarn")
+    text = log_capture.getvalue()
+    assert "INFO" in text
+    assert "save_connection enter" in text
+    assert "DEBUG" in text
+    assert "connection saved" in text
+
+
+def test_prepare_submit_job_emits_debug_and_info(log_capture):
+    connections.save_connection(name="prod", master="yarn", deploy_mode="cluster")
+    log_capture.truncate(0); log_capture.seek(0)
+    submit.prepare_submit_job(connection="prod", script_path="/tmp/j.py", queue="research")
+    text = log_capture.getvalue()
+    assert "DEBUG|prepare_submit_job enter" in text
+    assert "INFO|prepare_submit_job ok" in text
+    assert "script_path=/tmp/j.py" in text
+    assert "queue=research" in text
+
+
+def test_get_unknown_pending_job_emits_debug(log_capture):
+    log_capture.truncate(0); log_capture.seek(0)
+    import pytest as _pytest
+    with _pytest.raises(KeyError):
+        submit.get_pending_job("p_doesnotexist")
+    text = log_capture.getvalue()
+    assert "DEBUG|get_pending_job enter" in text
+    assert "p_doesnotexist" in text