feat: add enum discriminator support and identifier validation

This addresses additional code review feedback (Priority 2 items):

1. Add single-value enum discriminator support
   - Previously only handled "const" discriminators
   - Now also supports "enum": ["value"] as discriminator
   - Enables semantic naming for more schema patterns

2. Add identifier validation in snake_to_pascal()
   - Validates result is a valid Python identifier
   - Raises clear error if conversion produces invalid name
   - Prevents silent failures with unicode/special characters

Why these are easy wins:
- Enum support: 3-line addition, handles edge case
- Identifier validation: 5-line addition, defensive programming
- Both improve robustness with minimal complexity

Example: If a schema has discriminator like:
  "status": {"enum": ["active"]}  ← now detected same as const

Addresses code review Priority 2 items from code-reviewer agent.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: bokelley

scripts/generate_models_simple.py

@@ -28,8 +28,21 @@
 
 
 def snake_to_pascal(name: str) -> str:
-    """Convert snake_case to PascalCase."""
-    return "".join(word.capitalize() for word in name.split("-"))
+    """
+    Convert snake_case to PascalCase.
+
+    Raises:
+        ValueError: If the result is not a valid Python identifier
+    """
+    pascal = "".join(word.capitalize() for word in name.split("-"))
+
+    # Validate result is a valid Python identifier
+    if not pascal.isidentifier():
+        raise ValueError(
+            f"Cannot convert '{name}' to valid Python identifier: '{pascal}'"
+        )
+
+    return pascal
 
 
 def sanitize_field_name(name: str) -> str:
@@ -100,6 +113,10 @@ def generate_discriminated_union(schema: dict, base_name: str) -> str:
                     if "const" in disc_prop:
                         discriminator_value = disc_prop["const"]
                         break
+                    elif "enum" in disc_prop and len(disc_prop["enum"]) == 1:
+                        # Single-value enum can also be a discriminator
+                        discriminator_value = disc_prop["enum"][0]
+                        break
 
         # Generate variant name
         if discriminator_value: