[Fix] Include all SpendLogsMetadata keys in spend logs payload

The dict comprehension in _get_spend_logs_metadata was only including
metadata keys that existed in the input dict. After user_api_key_project_id
was added to SpendLogsMetadata, payloads missing that key in input would
not include it in output, causing test_spend_logs_payload to fail.

Use metadata.get(key) instead of filtering with `if key in metadata`
to ensure all SpendLogsMetadata keys are always present (defaulting to
None), consistent with the metadata-is-None branch.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

litellm/proxy/spend_tracking/spend_tracking_utils.py

@@ -96,9 +96,8 @@ def _get_spend_logs_metadata(
     # Filter the metadata dictionary to include only the specified keys
     clean_metadata = SpendLogsMetadata(
         **{  # type: ignore
-            key: metadata[key]
+            key: metadata.get(key)
             for key in SpendLogsMetadata.__annotations__.keys()
-            if key in metadata
         }
     )
     clean_metadata["applied_guardrails"] = applied_guardrails