feat(job_writer): write PySpark code to disk with env-var override

spark_executor/core/job_writer.py provides write_job_file(code, jobs_dir=None)
that resolves the output directory in this order:
  1. explicit jobs_dir argument (for tests + programmatic override)
  2. SPARK_EXECUTOR_JOBS_DIR environment variable (operator override)
  3. DEFAULT_JOBS_DIR = './data/jobs' (gitignored, persists via volume mount)

Filenames are job_<UTC-stamp>_<random-hex>.py — same-second writes still
get distinct names via the random suffix, so concurrent agents won't
clobber each other.

8 tests cover: arg>env>default precedence, dir auto-creation, env-var
override, default fallback in a tmp cwd, and uniqueness under concurrent
calls.

tests/unit/test_job_writer.py

@@ -0,0 +1,78 @@
+# coding=utf-8
+import os
+from pathlib import Path
+
+from spark_executor.core.job_writer import (
+    DEFAULT_JOBS_DIR,
+    ENV_JOBS_DIR,
+    resolve_jobs_dir,
+    write_job_file,
+)
+
+
+# --- resolve_jobs_dir (priority: arg > env > default) ---
+
+def test_resolve_explicit_arg_wins(tmp_path: Path, monkeypatch):
+    monkeypatch.setenv(ENV_JOBS_DIR, "/from/env")
+    assert resolve_jobs_dir(str(tmp_path)) == str(tmp_path)
+
+
+def test_resolve_env_var_used_when_no_arg(monkeypatch, tmp_path: Path):
+    monkeypatch.setenv(ENV_JOBS_DIR, str(tmp_path))
+    assert resolve_jobs_dir() == str(tmp_path)
+
+
+def test_resolve_default_when_neither_set(monkeypatch):
+    monkeypatch.delenv(ENV_JOBS_DIR, raising=False)
+    assert resolve_jobs_dir() == DEFAULT_JOBS_DIR
+    assert resolve_jobs_dir() == "./data/jobs"
+
+
+# --- write_job_file ---
+
+def test_write_creates_file_with_code_and_returns_abs_path(tmp_path: Path):
+    out = write_job_file("print('hi')\n", jobs_dir=str(tmp_path))
+    assert os.path.isfile(out)
+    assert os.path.isabs(out)
+    assert out.startswith(str(tmp_path))
+    assert out.endswith(".py")
+    with open(out) as f:
+        assert f.read() == "print('hi')\n"
+
+
+def test_write_creates_jobs_dir_if_missing(tmp_path: Path):
+    target = tmp_path / "newdir"
+    assert not target.exists()
+    out = write_job_file("x = 1\n", jobs_dir=str(target))
+    assert target.is_dir()
+    assert os.path.isfile(out)
+
+
+def test_write_uses_env_var_when_no_arg(monkeypatch, tmp_path: Path):
+    monkeypatch.setenv(ENV_JOBS_DIR, str(tmp_path))
+    out = write_job_file("env-driven\n")
+    assert out.startswith(str(tmp_path))
+    with open(out) as f:
+        assert f.read() == "env-driven\n"
+
+
+def test_write_uses_default_when_neither_set(monkeypatch, tmp_path: Path):
+    """Default is ./data/jobs relative to cwd. Run in a tmp dir so the test
+    doesn't pollute the real project and stays self-contained."""
+    monkeypatch.delenv(ENV_JOBS_DIR, raising=False)
+    monkeypatch.chdir(tmp_path)  # pytest built-in: chdir for this test only
+    out = write_job_file("default\n")
+    assert os.path.isabs(out)
+    # Should resolve to <tmp_path>/data/jobs/job_*.py
+    expected_dir = tmp_path / "data" / "jobs"
+    assert expected_dir.is_dir()
+    assert str(out).startswith(str(expected_dir))
+    with open(out) as f:
+        assert f.read() == "default\n"
+
+
+def test_write_returns_unique_paths_for_concurrent_calls(tmp_path: Path):
+    """Two writes in the same second must still get distinct filenames (via random suffix)."""
+    out1 = write_job_file("a", jobs_dir=str(tmp_path))
+    out2 = write_job_file("b", jobs_dir=str(tmp_path))
+    assert out1 != out2