fix: validate content items and require data field in binary content (#5238)

## Problem

Two unit tests in `internal/mcp/tool_result_test.go` fail:

- **`TestDecodeContentData/missing_image_data_returns_error`** —
`decodeContentData` silently returns `nil, nil` when the `data` field is
missing, but the test expects an error.
- **`TestConvertMapToCallToolResult_ContentItemCastFailure`** — non-map
entries in `[]interface{}` content arrays are silently dropped instead
of causing an error.

## Fix

1. **`decodeContentData`**: Now checks for the presence of the `data`
key and returns a descriptive error when it's missing or nil, preventing
corrupted backend responses from surfacing as empty media.

2. **`ConvertToCallToolResult`**: Now rejects non-map entries in
`[]interface{}` content arrays with an error instead of silently
dropping them, preventing data loss from malformed responses.

## Verification

`make agent-finished` passes — all unit and integration tests green.

Author: lpcox

internal/mcp/tool_result.go

@@ -72,10 +72,12 @@ func convertMapToCallToolResult(m map[string]interface{}) (*sdk.CallToolResult,
 	switch v := contentVal.(type) {
 	case []interface{}:
 		items = make([]map[string]interface{}, 0, len(v))
-		for _, item := range v {
-			if ci, ok := item.(map[string]interface{}); ok {
-				items = append(items, ci)
+		for i, item := range v {
+			ci, ok := item.(map[string]interface{})
+			if !ok {
+				return nil, fmt.Errorf("content item %d: expected map, got %T", i, item)
 			}
+			items = append(items, ci)
 		}
 	case []map[string]interface{}:
 		items = v
@@ -155,13 +157,17 @@ func convertContentItem(ci map[string]interface{}) (sdk.Content, error) {
 // When data arrives via json.Unmarshal into interface{}, []byte fields are stored as
 // base64 strings; this function handles both the string and pre-decoded []byte forms.
 func decodeContentData(ci map[string]interface{}) ([]byte, error) {
-	switch v := ci["data"].(type) {
+	raw, exists := ci["data"]
+	if !exists || raw == nil {
+		return nil, fmt.Errorf("missing required 'data' field")
+	}
+	switch v := raw.(type) {
 	case []byte:
 		return v, nil
 	case string:
 		return base64.StdEncoding.DecodeString(v)
 	default:
-		return nil, nil
+		return nil, fmt.Errorf("unsupported data type %T", raw)
 	}
 }