chore(python): pre-merge simplifier pass — FR5a polish

- errors.py: unquote `Optional[ErrorSource]` annotation (redundant under
  `from __future__ import annotations`; matches sibling annotations in
  this file).
- error_source.py: unquote `ClassVar[CodeSource]` self-reference (same
  reason) and trim the verbose multi-line forward-declaration comment
  to a single-line WHY note.

Conservative diff: two annotation cleanups + one comment trim. No
behavior change; 496/496 Python tests stay green. Lazy import in
semantic_diff was verified necessary (eager import reproduces the
circular-import ImportError) and left as-is.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dmealing

server/python/src/metaobjects/errors.py

@@ -66,7 +66,7 @@ def __init__(
         code: ErrorCode = ErrorCode.ERR_UNKNOWN,
         source: str | None = None,
         path: str | None = None,
-        envelope: "Optional[ErrorSource]" = None,
+        envelope: Optional[ErrorSource] = None,
     ) -> None:
         self.message = message
         self.code = code

server/python/src/metaobjects/source/error_source.py

@@ -147,12 +147,10 @@ class CodeSource(ErrorSource):
     caller: Optional[str] = None
     format: ClassVar[str] = "code"
 
-    # Canonical singleton for the no-caller case. Frozen dataclasses are
-    # hash-equal across instances, but a single shared default makes intent
-    # explicit and matches the C# `CodeSource.Default` convention. Declared
-    # here as a forward reference so static analyzers see `CodeSource.DEFAULT`
-    # as a `CodeSource` (the assignment lives below).
-    DEFAULT: ClassVar["CodeSource"]
+    # Canonical singleton for the no-caller case (mirrors C# `CodeSource.Default`).
+    # The actual assignment lives below — Python doesn't allow self-reference in
+    # the dataclass body.
+    DEFAULT: ClassVar[CodeSource]
 
 
 CodeSource.DEFAULT = CodeSource(caller=None)