Fix Python 3.9 compatibility: add from __future__ import annotations

Three toolchain files use Python 3.10+ type union syntax (int | None,
str | None) which pylint flags as E1131 when running under Python 3.9.
Add 'from __future__ import annotations' to enable the new syntax on
Python 3.9+.

Files fixed:
- toolchain/mfc/gen_case_constraints_docs.py
- toolchain/mfc/params/ast_analyzer.py
- toolchain/mfc/params/generators/docs_gen.py

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Spencer Bryngelson

toolchain/mfc/gen_case_constraints_docs.py

@@ -8,6 +8,8 @@
 Also generates case design playbook from curated working examples.
 """  # pylint: disable=too-many-lines
 
+from __future__ import annotations
+
 import json
 import sys
 import subprocess

toolchain/mfc/params/ast_analyzer.py

@@ -7,6 +7,8 @@
 and case_constraints.md).
 """
 
+from __future__ import annotations
+
 import ast
 import re
 from dataclasses import dataclass, field

toolchain/mfc/params/generators/docs_gen.py

@@ -5,6 +5,8 @@
 organized by family with descriptions, types, and constraints.
 """
 
+from __future__ import annotations
+
 from typing import Any, Dict, List, Tuple
 from collections import defaultdict
 import re