[test-improver] Improve tests for mcp/tool_result (#5788)

# Test Improvements: `tool_result_test.go`

## File Analyzed

- **Test File**: `internal/mcp/tool_result_test.go`
- **Package**: `internal/mcp`
- **Implementation**: `internal/mcp/tool_result.go`

## Improvements Made

### 1. Increased Coverage
- ✅ Added test for audio content with missing `data` field
- ✅ Added test for image content with unsupported data type (e.g.,
`int`) — covers `decodeContentData` `default:` branch
- ✅ Added test for image content with `nil` data value
- ✅ Added test for resource content where `json.Unmarshal` into
`sdk.ResourceContents` fails
- **Previous Coverage**: `decodeContentData` 85.7%, `convertContentItem`
96.4%
- **New Coverage**: `decodeContentData` **100%**, `convertContentItem`
**100%**

### 2. Better Testing Patterns
- ✅ New tests use existing testify assertion patterns (`assert.Error`,
`assert.ErrorContains`, `assert.Nil`) consistent with the file's style
- ✅ Descriptive subtest names explain the invariant being asserted
- ✅ Comments explain why each case is important (e.g., guarding against
corrupted backend responses)

### 3. Cleaner & More Stable Tests
- ✅ Each subtest is independent with no shared state
- ✅ Tests use inline data, no external dependencies

## Test Execution

All tests pass:
```
ok  	github.com/github/gh-aw-mcpg/internal/mcp	2.325s
```

Coverage improvement in `tool_result.go`:
```
tool_result.go:145: decodeContentData   85.7% → 100.0%
tool_result.go:98:  convertContentItem  96.4% → 100.0%
```

## Why These Changes?

`tool_result.go` is central to MCP response handling — it converts every
backend tool result into the SDK format. The uncovered branches
represent real error conditions (unsupported data types, unmarshalable
resources) that could surface from misbehaving backends. Adding these
tests ensures the error paths are exercised and their error messages are
validated.

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*Focuses on better patterns, increased coverage, and more stable tests*




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Author: lpcox

internal/mcp/tool_result_test.go

@@ -507,6 +507,44 @@ func TestDecodeContentData(t *testing.T) {
 		assert.Nil(t, result)
 		assert.ErrorContains(t, err, "failed to decode audio data")
 	})
+
+	t.Run("missing audio data field returns error", func(t *testing.T) {
+		ci := map[string]interface{}{
+			"type":     "audio",
+			"mimeType": "audio/wav",
+			// no "data" key
+		}
+		result, err := convertContentItem(ci)
+		assert.Error(t, err, "missing audio data should produce an error")
+		assert.Nil(t, result)
+		assert.ErrorContains(t, err, "failed to decode audio data")
+	})
+
+	t.Run("unsupported data type for image returns error", func(t *testing.T) {
+		// decodeContentData must reject types that are neither []byte nor string.
+		ci := map[string]interface{}{
+			"type":     "image",
+			"mimeType": "image/png",
+			"data":     42, // integer — not a valid data carrier
+		}
+		result, err := convertContentItem(ci)
+		assert.Error(t, err, "unsupported data type should produce an error")
+		assert.Nil(t, result)
+		assert.ErrorContains(t, err, "failed to decode image data")
+	})
+
+	t.Run("nil data value in image content returns error", func(t *testing.T) {
+		// data key exists but is nil — decodeContentData must treat this as missing.
+		ci := map[string]interface{}{
+			"type":     "image",
+			"mimeType": "image/png",
+			"data":     nil,
+		}
+		result, err := convertContentItem(ci)
+		assert.Error(t, err, "nil data value should produce an error")
+		assert.Nil(t, result)
+		assert.ErrorContains(t, err, "failed to decode image data")
+	})
 }
 
 // TestConvertContentItem_ResourceMarshalError verifies that an unmarshalable
@@ -523,6 +561,23 @@ func TestConvertContentItem_ResourceMarshalError(t *testing.T) {
 	assert.ErrorContains(t, err, "failed to marshal resource")
 }
 
+// TestConvertContentItem_ResourceUnmarshalError verifies that resource JSON that
+// cannot be unmarshaled into sdk.ResourceContents returns an appropriate error.
+func TestConvertContentItem_ResourceUnmarshalError(t *testing.T) {
+	// Provide a resource value where a required string field contains an object,
+	// which will marshal fine but fail to unmarshal into sdk.ResourceContents.
+	ci := map[string]interface{}{
+		"type": "resource",
+		"resource": map[string]interface{}{
+			"uri": map[string]interface{}{"nested": "object"}, // URI must be a string
+		},
+	}
+	result, err := convertContentItem(ci)
+	assert.Error(t, err)
+	assert.Nil(t, result)
+	assert.ErrorContains(t, err, "failed to parse resource")
+}
+
 // TestConvertMapToCallToolResult_ContentItemCastFailure verifies that
 // malformed []interface{} content entries are rejected instead of being
 // silently skipped, to avoid data loss.