fix(optimization): pin LD-API-Version on agent-optimization API calls (AIC-3484) (#216)

## Summary
- Agent-optimization requests now send `LD-API-Version: 20240415` by
default.
- AI Config calls in the same client still send `beta`, which overrides
that default.
- After 2026-12-31, a call with no version header is rejected when the
customer's token defaults below API v4. These requests use that token.

## Test plan
- [x] `uv run pytest` in `packages/optimization`
- [x] Confirm a request with no extra headers records `Ld-api-version:
20240415`, and `get_ai_config_variation` still sends `beta`.

Author: jeffdupont

packages/optimization/src/ldai_optimizer/ld_api_client.py

@@ -198,7 +198,10 @@ def __repr__(self) -> str:
         return f"LDApiClient(base_url={self._base_url!r})"
 
     def _auth_headers(self) -> Dict[str, str]:
-        return {"Authorization": self._api_key}
+        return {
+            "Authorization": self._api_key,
+            "LD-API-Version": "20240415",
+        }
 
     def _request(
         self,

packages/optimization/tests/test_ld_api_client.py

@@ -168,6 +168,14 @@ def test_authorization_header_always_sent(self):
             req: urllib.request.Request = mock_open.call_args[0][0]
             assert req.get_header("Authorization") == "my-api-key"
 
+    def test_default_api_version_header_is_v4(self):
+        client = LDApiClient("test-key")
+        with patch("urllib.request.urlopen", return_value=_mock_urlopen({})) as mock_open:
+            client._request("GET", "/path")
+            req: urllib.request.Request = mock_open.call_args[0][0]
+            # urllib capitalizes header names; the wire value is still LD-API-Version.
+            assert req.get_header("Ld-api-version") == "20240415"
+
     def test_raises_ld_api_error_on_http_error(self):
         client = LDApiClient("test-key")
         http_error = urllib.error.HTTPError(