Resolve python_version from language_version for backward compat (#2488)

## Summary

- Add `model_validator` to `OptimizeSchema`, `OptimizeSchemaLP`, and
`TestGenSchema` that resolves `python_version` from `language_version`
when the request language is `"python"`
- Old clients sending `python_version` continue to work unchanged
- New clients sending only `language_version` get `python_version`
populated automatically for downstream handlers

This must be deployed **before** the client-side changes that stop
sending `python_version`
(codeflash-ai/codeflash#remove-python-version-field).

Once all clients are updated, `python_version` can be fully removed from
the schemas and handlers.

## Test plan

- [x] Existing schema tests pass
(`tests/optimizer/test_javascript_schema.py`)
- [ ] Verify old clients (sending `python_version`) still work after
deploy
- [ ] Verify new clients (sending only `language_version`) resolve
correctly

django/aiservice/core/shared/optimizer_models.py

@@ -3,8 +3,10 @@
 from __future__ import annotations
 
 import enum
+from typing import Self
 
 from ninja import Schema
+from pydantic import model_validator
 
 
 class OptimizedCandidateSource(str, enum.Enum):
@@ -34,6 +36,13 @@ class OptimizeSchema(Schema):
     n_candidates: int = 5  # default value for backward compatibility
     is_numerical_code: bool | None = None
 
+    @model_validator(mode="after")
+    def resolve_python_version(self) -> Self:
+        """Resolve python_version from language_version for backward compatibility."""
+        if self.python_version is None and self.language_version is not None and self.language == "python":
+            self.python_version = self.language_version
+        return self
+
 
 class OptimizeSchemaLP(Schema):
     source_code: str
@@ -49,3 +58,10 @@ class OptimizeSchemaLP(Schema):
     model: str | None = None
     call_sequence: int | None = None
     is_numerical_code: bool | None = None
+
+    @model_validator(mode="after")
+    def resolve_python_version(self) -> Self:
+        """Resolve python_version from language_version for backward compatibility."""
+        if self.python_version is None and self.language_version is not None and self.language == "python":
+            self.python_version = self.language_version
+        return self

django/aiservice/core/shared/testgen_models.py

@@ -28,6 +28,13 @@ class TestGenSchema(Schema):
     is_numerical_code: bool | None = None
     module_system: str | None = None  # "esm" or "commonjs" (JS/TS only)
 
+    @model_validator(mode="after")
+    def resolve_python_version(self) -> Self:
+        """Resolve python_version from language_version for backward compatibility."""
+        if self.python_version is None and self.language_version is not None and self.language == "python":
+            self.python_version = self.language_version
+        return self
+
     @model_validator(mode="after")
     def helper_function_names_validator(self) -> Self:
         # To maintain backwards compatibility