fix(value-source): preserve store contract for game_event + error precedence

Two HIGH issues surfaced by review of 3b8f00e:

1. ``_build_game_event`` was newly succeeding where the old store
   raised ``ValueError("Invalid source type: game_event")``. The
   coverage-assertion-symmetry comment was honest about it being
   a path that didn't exist before, but silent broadening of the
   create contract is a real behaviour delta — any internal caller
   that previously caught the error would now succeed.

   Make ``_build_game_event`` raise NotImplementedError. The
   coverage assertion still passes (the entry exists), but the
   historical "you can't create game_event sources through the
   store" contract is preserved. game_event instances continue to
   be wired up by the game-integration setup path.

2. The new ``create_source`` ran ``_check_name_unique`` BEFORE
   ``build_source``. When both ``source_type`` is invalid AND
   ``name`` collides with an existing source, the old code raised
   ``"Invalid source type: …"`` first; the new code raised the
   name-collision error. Swap the order: build first (which
   validates source_type), then check name uniqueness, then
   persist. Bonus: a uuid is no longer minted for a source we end
   up rejecting on type.

New test pins the game_event NotImplementedError so a future
refactor doesn't accidentally re-open the create path.

38 value-source-store + factory tests stay green; ruff clean.

server/tests/storage/test_value_source_factories.py

@@ -180,6 +180,26 @@ def test_build_source_unknown_type_raises():
         )
 
 
+def test_build_source_game_event_is_not_creatable_via_store():
+    """game_event sources were never creatable through the store API.
+
+    The builder exists for coverage-assertion symmetry but raises so the
+    historical contract holds (game-event value sources are wired up by
+    the game-integration setup path, not user CRUD).
+    """
+    with pytest.raises(NotImplementedError, match="game_event"):
+        vsf.build_source(
+            source_type="game_event",
+            sid="vs_t",
+            name="Test",
+            now=_now(),
+            description=None,
+            tags=None,
+            icon=None,
+            icon_color=None,
+        )
+
+
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