Fix schema generator to use union operator for consistency

[bokelley]

Problem

The schema generation script was missing the --use-union-operator flag, causing it to regenerate all files with Optional[X] instead of X | None syntax whenever it ran. This created constant drift between committed schemas and regenerated ones.

Changes

• ✅ Add --use-union-operator flag to datamodel-codegen command
• ✅ Ensures generated schemas use modern Python 3.10+ syntax (X | None)
• ✅ Prevents 70+ file changes every time schema generation runs
• ✅ Fixes line wrapping inconsistencies in 14 schema files
• ✅ Remove broken check-generated-schemas pre-commit hook (script never existed)

Root Cause

PR #572 committed schemas with X | None syntax, but the generator script still produced Optional[X] syntax, creating constant drift between committed files and regenerated files.

Testing

• ✅ Regenerated schemas produce zero git changes
• ✅ All unit tests pass (845 passed, 21 skipped)
• ✅ All integration tests pass (174 passed, 75 skipped)
• ✅ Pre-commit hooks pass

Verification

# Before fix: 70+ files changed
git status src/core/schemas_generated/

# After fix: 0 files changed
uv run python scripts/generate_schemas.py
git status src/core/schemas_generated/  # Clean!