fix(mypy): resolve provider utility and testing type issues (#3935)

Fixes mypy type errors in provider utilities and testing infrastructure:
- `mcp.py`: Cast incompatible client types, wrap image data properly
- `batches.py`: Rename walrus variable to avoid shadowing
- `api_recorder.py`: Use cast for Pydantic field annotation

No functional changes.

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Co-authored-by: Claude <noreply@anthropic.com>

src/llama_stack/models/llama/llama3/multimodal/encoder_utils.py

@@ -141,7 +141,7 @@ def build_encoder_attention_mask(
     """
     Build vision encoder attention mask that omits padding tokens.
     """
-    masks_list = []
+    masks_list: list[torch.Tensor] = []
     for arx in ar:
         mask_i = torch.ones((num_chunks, x.shape[2], 1), dtype=x.dtype)
         mask_i[: arx[0] * arx[1], :ntok] = 0