test(stock): retain cross-process ID regression from PR OpenMontageInc#815

Adapted from PR OpenMontageInc#815 by @serdarucel: retain deterministic shape and PYTHONHASHSEED subprocess coverage with canonical PR OpenMontageInc#597 stable_url_id (16 hexadecimal characters). The cohort preserves the original reviewed heads for OpenMontageInc#594, OpenMontageInc#595, OpenMontageInc#597, OpenMontageInc#639, OpenMontageInc#652, OpenMontageInc#653, OpenMontageInc#779 and OpenMontageInc#857. No audio or Pixabay retry implementations are included.

Author: OpenMontage Review

tests/tools/test_stock_source_stable_id.py

@@ -0,0 +1,45 @@
+"""Issue #596 regression: source_id must be stable across processes.
+
+Adapted from PR #815 by @serdarucel:
+https://github.com/calesthio/OpenMontage/pull/815
+Retains cross-process coverage with PR #597's shared 64-bit stable_url_id.
+"""
+
+import os
+from pathlib import Path
+import subprocess
+import sys
+
+from tools.video.stock_sources.base import stable_url_id
+
+
+def test_stable_url_id_is_deterministic():
+    url = "https://example.com/x"
+    source_id = stable_url_id(url)
+
+    assert source_id == stable_url_id(url)
+    assert len(source_id) == 16
+    assert all(char in "0123456789abcdef" for char in source_id)
+
+
+def test_stable_url_id_matches_subprocess_with_random_seed():
+    repo_root = Path(__file__).resolve().parents[2]
+    command = (
+        "from tools.video.stock_sources.base import stable_url_id; "
+        "print(stable_url_id('https://example.com/x'))"
+    )
+    outputs = []
+    for seed in ("1", "2"):
+        env = os.environ.copy()
+        env["PYTHONHASHSEED"] = seed
+        outputs.append(
+            subprocess.check_output(
+                [sys.executable, "-c", command],
+                cwd=repo_root,
+                env=env,
+                text=True,
+                timeout=15,
+            ).strip()
+        )
+
+    assert outputs[0] == outputs[1] == stable_url_id("https://example.com/x")