fix(action/schema): correct validate_name error message to say "valid Python identifier" and remove "or hyphens" claim

Update ActionArgumentSchema.validate_name to:
- Use v.isidentifier() instead of v.replace('-', '_').isidentifier() so
  hyphens are correctly rejected as invalid argument names
- Replace the misleading error message 'Use alphanumeric characters,
  underscores, or hyphens' with 'Argument name must be a valid Python
  identifier' to match the BDD spec expectation exactly
- Remove the phrase 'or hyphens' which incorrectly implied hyphens were
  valid argument names

ISSUES CLOSED: #3039

src/cleveragents/action/schema.py

@@ -122,11 +122,12 @@ class ActionArgumentSchema(BaseModel):
     @field_validator("name")
     @classmethod
     def validate_name(cls, v: str) -> str:
-        """Ensure argument name is a valid identifier."""
-        if not v.replace("-", "_").isidentifier():
+        """Ensure argument name is a valid Python identifier."""
+        if not v.isidentifier():
             raise ValueError(
-                f"Argument name '{v}' is not a valid identifier. "
-                "Use alphanumeric characters, underscores, or hyphens."
+                f"Argument name '{v}' is not a valid Python identifier. "
+                "Argument name must be a valid Python identifier "
+                "(alphanumeric and underscores, not starting with a digit)."
             )
         return v