feat: env-configurable spark-submit binary name

Hardcoding 'spark-submit' as cmd[0] in build_spark_submit_command
breaks for hosts where:
  - both Spark 1.x and 2.x/3.x are installed and 'spark-submit' resolves
    to the wrong one (use 'spark2-submit' or 'spark3-submit' explicitly)
  - the user wants to launch via the PySpark entrypoint ('pyspark')
  - a custom wrapper script sits on PATH (e.g. a credentials-injecting
    'spark-submit-wrapper')

New env var SPARK_EXECUTOR_SPARK_SUBMIT_BIN. Default is 'spark-submit'
(preserves the current behavior for everyone). Override in .env /
docker-compose.yml to change.

common/config.py:
  - new Settings.spark_submit_bin field
  - env-var resolution in from_env() with default 'spark-submit'
  - included in reload() so tests work

spark_executor/core/spark_submit.py:
  - cmd[0] reads settings.spark_submit_bin (was hardcoded 'spark-submit')

.env.example: new section with the override and example values.
docker-compose.yml: forwards the var with the standard 'spark-submit'
default.

Tests: 2 new (settings.spark_submit_bin='spark2-submit', ='pyspark')
plus existing tests updated to use the settings-restore fixture so
mutations don't leak between tests.

165/163 still pass.

common/config.py

@@ -65,6 +65,17 @@ class Settings:
     # file sink always captures DEBUG (full audit trail).
     log_level: str = "DEBUG"
 
+    # --- Spark CLI binary name ---
+    # The program name used in cmd[0] when invoking the Spark client.
+    # Defaults to 'spark-submit' (the standard Spark 2.x / 3.x / 4.x CLI).
+    # Override via SPARK_EXECUTOR_SPARK_SUBMIT_BIN for:
+    #   - 'spark2-submit' on a system where both Spark 1.x and 2.x are
+    #     installed and spark-submit points to the wrong one
+    #   - a custom wrapper script (e.g. '/usr/local/bin/spark-submit-wrapper')
+    #   - 'pyspark' if you want to launch via the PySpark entrypoint
+    # The binary is resolved against $PATH (set by docker-entrypoint.sh).
+    spark_submit_bin: str = "spark-submit"
+
     @classmethod
     def from_env(cls) -> "Settings":
         data_dir = os.environ.get("SPARK_EXECUTOR_DATA_DIR", "./data")
@@ -81,6 +92,9 @@ class Settings:
             # `or None` collapses empty string to None for the URL fallback
             yarn_resource_manager_url=os.environ.get("YARN_RESOURCE_MANAGER_URL") or None,
             log_level=os.environ.get("SPARK_EXECUTOR_LOG_LEVEL", "DEBUG"),
+            spark_submit_bin=os.environ.get(
+                "SPARK_EXECUTOR_SPARK_SUBMIT_BIN", "spark-submit"
+            ),
         )
 
     def reload(self) -> "Settings":
@@ -95,6 +109,7 @@ class Settings:
         self.log_dir = fresh.log_dir
         self.yarn_resource_manager_url = fresh.yarn_resource_manager_url
         self.log_level = fresh.log_level
+        self.spark_submit_bin = fresh.spark_submit_bin
         return self