test: add BOM fixture coverage for Python/Ruby AST serializers

[rhuanbarreto]

Context

From PR #452 review: the Python (utf-8-sig) and Ruby (r:bom|utf-8) BOM-stripping codecs are not directly tested with BOM-bearing fixtures. A regression in these encoding settings would slip through undetected.

Proposal

Add end-to-end tests that write fixtures with a UTF-8 BOM prefix (\xEF\xBB\xBF) and verify ctx.ast() parses them correctly for both Python and Ruby. This can go in either tests/engine/ast-support.test.ts (serializer-level) or tests/engine/runner-ast.test.ts (integration-level).

References

• PR feat: ast-aware rule context (ctx.ast) for TS/JS/Python/Ruby #452 review thread on BOM coverage
• Files: tests/engine/ast-support.test.ts, tests/engine/runner-ast.test.ts
• Serializer programs: PYTHON_AST_PROGRAM (utf-8-sig), RUBY_AST_PROGRAM (r:bom|utf-8)