Replace custom template substitution with yt-dlp's evaluate_outtmpl

Replace the hand-rolled _outtmpl_substitute_field() / _compile_outtmpl_pattern()
with a new _resolve_outtmpl_fields() that delegates to yt-dlp's
YoutubeDL.evaluate_outtmpl().  This gives playlist/channel output templates
access to yt-dlp's full template syntax: defaults (%(field|fallback)s),
conditional formatting (%(field&prefix {})s), math (%(field+N)d),
datetime formatting (%(field>%Y-%m-%d)s), and more.

Only field references whose root name matches the targeted prefix (e.g.
"playlist" or "channel") are resolved; all other references remain as
template placeholders for yt-dlp to fill during the actual download.

Agent-Logs-Url: https://github.com/alexta69/metube/sessions/0ae5ff34-540f-4fc8-a81c-358fb92b7c15

Co-authored-by: alexta69 <7450369+alexta69@users.noreply.github.com>

app/tests/test_ytdl_utils.py

@@ -24,7 +24,7 @@ class _ImpersonateTarget:
 
 fake_impersonate.ImpersonateTarget = _ImpersonateTarget
 fake_networking.impersonate = fake_impersonate
-fake_utils.STR_FORMAT_RE_TMPL = r"(?P<prefix>)%\((?P<has_key>{})\)(?P<format>[-0-9.]*{})"
+fake_utils.STR_FORMAT_RE_TMPL = r"(?P<prefix>)%\((?P<has_key>(?P<key>{}))\)(?P<format>[-0-9.]*{})"
 fake_utils.STR_FORMAT_TYPES = "diouxXeEfFgGcrsa"
 fake_yt_dlp.networking = fake_networking
 fake_yt_dlp.utils = fake_utils
@@ -37,11 +37,15 @@ from ytdl import (
     DownloadInfo,
     _compact_persisted_entry,
     _convert_srt_to_txt_file,
-    _outtmpl_substitute_field,
+    _resolve_outtmpl_fields,
     _sanitize_entry_for_pickle,
     _sanitize_path_component,
 )
 
+# Detect whether the real yt-dlp is loaded (as opposed to the minimal fake
+# shim above).  _resolve_outtmpl_fields needs YoutubeDL at runtime.
+_has_real_ytdlp = hasattr(sys.modules.get("yt_dlp"), "YoutubeDL")
+
 
 class SanitizePathComponentTests(unittest.TestCase):
     def test_replaces_windows_invalid_chars(self):
@@ -52,15 +56,39 @@ class SanitizePathComponentTests(unittest.TestCase):
         self.assertEqual(_sanitize_path_component(42), 42)
 
 
-class OuttmplSubstituteFieldTests(unittest.TestCase):
-    def test_simple_substitution(self):
-        self.assertEqual(_outtmpl_substitute_field("%(title)s", "title", "Hello"), "Hello")
+@unittest.skipUnless(_has_real_ytdlp, "requires real yt-dlp")
+class ResolveOuttmplFieldsTests(unittest.TestCase):
+    """Tests for _resolve_outtmpl_fields (delegates to yt-dlp's template engine)."""
+
+    def test_simple_playlist_substitution(self):
+        info = {"playlist_title": "My PL", "playlist_index": "03"}
+        result = _resolve_outtmpl_fields("%(playlist_title)s/%(title)s.%(ext)s", info, ("playlist",))
+        self.assertEqual(result, "My PL/%(title)s.%(ext)s")
 
     def test_format_spec_int(self):
-        self.assertEqual(_outtmpl_substitute_field("%(idx)02d", "idx", 3), "03")
+        info = {"playlist_index": "3"}
+        result = _resolve_outtmpl_fields("%(playlist_index)02d-%(title)s", info, ("playlist",))
+        self.assertEqual(result, "03-%(title)s")
 
-    def test_missing_field_unchanged(self):
-        self.assertEqual(_outtmpl_substitute_field("%(other)s", "title", "x"), "%(other)s")
+    def test_non_targeted_fields_unchanged(self):
+        info = {"playlist_title": "PL"}
+        result = _resolve_outtmpl_fields("%(title)s/%(ext)s", info, ("playlist",))
+        self.assertEqual(result, "%(title)s/%(ext)s")
+
+    def test_default_value(self):
+        info = {"playlist_index": "1"}
+        result = _resolve_outtmpl_fields("%(playlist_title|Unknown)s/%(playlist_index)s", info, ("playlist",))
+        self.assertEqual(result, "Unknown/1")
+
+    def test_channel_prefix(self):
+        info = {"channel": "MyChan", "channel_index": "05"}
+        result = _resolve_outtmpl_fields("%(channel)s/%(channel_index)02d-%(title)s", info, ("channel",))
+        self.assertEqual(result, "MyChan/05-%(title)s")
+
+    def test_math_operation(self):
+        info = {"playlist_index": "3"}
+        result = _resolve_outtmpl_fields("%(playlist_index+100)d", info, ("playlist",))
+        self.assertEqual(result, "103")
 
 
 class SanitizeEntryForPickleTests(unittest.TestCase):