fix(admin): show the rate that needs fixing in the single-model view too

The listing already renders a stored non-finite rate as text rather than
letting the encoder report it as `null`, indistinguishable from a rate the row
never carried. The single-model view is the other view of that same row — the
one an operator opens from the listing to repair it — and still answered
`null`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011cKHVF5LA7TR5QuYi6ErLM

routstr/core/admin.py

@@ -682,9 +682,13 @@ async def get_provider_model(provider_id: str, model_id: str) -> dict[str, objec
             raise HTTPException(
                 status_code=404, detail="Model not found for this provider"
             )
-        return _row_to_model(
-            row, apply_provider_fee=False, provider_fee=provider.provider_fee
-        ).dict()  # type: ignore
+        # Same duty as the listing this view is opened from: a stored rate that
+        # is not a usable number must be shown as it is, not encoded as `null`.
+        return json_compliant(  # type: ignore[return-value]
+            _row_to_model(
+                row, apply_provider_fee=False, provider_fee=provider.provider_fee
+            ).dict()
+        )
 
 
 @admin_router.delete(