[test-improver] Improve tests for mcp/tool_result package (#4011)

## File Analyzed

- **Test File**: `internal/mcp/tool_result_test.go`
- **Package**: `internal/mcp`
- **Lines Added**: +60

## Improvements Made

### 1. New Coverage: `NewErrorCallToolResult`
The `NewErrorCallToolResult` function had **0% coverage** — it was
entirely untested. Added `TestNewErrorCallToolResult` with three
subtests:
- ✅ Non-nil error returns `IsError=true` result with the error message
as `TextContent`
- ✅ Nil error input substitutes `"unknown error"` message (covers the
nil-guard branch)
- ✅ Error messages with special characters (e.g. embedded JSON) are
preserved verbatim

### 2. New Coverage: `ConvertToCallToolResult` marshal error path
Added `TestConvertToCallToolResult_MarshalError` to cover the
early-return error path when the data argument cannot be JSON-marshaled
(e.g., passing a Go channel). This path was previously unreachable by
any test.

### 3. Coverage Improvement

| Function | Before | After |
|---|---|---|
| `NewErrorCallToolResult` | **0%** | **100%** |
| `ConvertToCallToolResult` | 89.7% | 93.1% |
| `internal/mcp` package total | 81.0% | 81.6% |

## Test Execution

All new and existing tests pass:

```
=== RUN   TestNewErrorCallToolResult
=== RUN   TestNewErrorCallToolResult/non-nil_error_produces_IsError_result_with_error_message_as_text
=== RUN   TestNewErrorCallToolResult/nil_error_substitutes_unknown_error_message
=== RUN   TestNewErrorCallToolResult/error_message_with_special_characters_is_preserved
--- PASS: TestNewErrorCallToolResult (0.00s)
=== RUN   TestConvertToCallToolResult_MarshalError
--- PASS: TestConvertToCallToolResult_MarshalError (0.00s)
PASS
ok  github.com/github/gh-aw-mcpg/internal/mcp  0.011s
```

## Why These Changes?

`NewErrorCallToolResult` is a utility used when a tool call fails and
the gateway needs to produce a structured error response back to the MCP
client. Despite being a public, used function, it had zero test
coverage. The nil-error guard and the function's three return values
(result, nil, err) are now all exercised.

The `ConvertToCallToolResult` marshal error path guards against
programming errors where an un-serializable type is accidentally passed
in; this defensive path was untested.

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Author: lpcox

internal/mcp/tool_result_test.go

@@ -2,6 +2,7 @@ package mcp
 
 import (
 	"encoding/json"
+	"errors"
 	"testing"
 
 	sdk "github.com/modelcontextprotocol/go-sdk/mcp"
@@ -315,3 +316,62 @@ func TestParseToolArguments(t *testing.T) {
 		assert.Empty(t, args)
 	})
 }
+
+// TestNewErrorCallToolResult tests construction of error CallToolResult values.
+func TestNewErrorCallToolResult(t *testing.T) {
+	t.Run("non-nil error produces IsError result with error message as text", func(t *testing.T) {
+		inputErr := errors.New("tool execution failed")
+
+		result, second, returnedErr := NewErrorCallToolResult(inputErr)
+
+		require.NotNil(t, result)
+		assert.True(t, result.IsError)
+		assert.Nil(t, second)
+		assert.ErrorIs(t, returnedErr, inputErr)
+
+		require.Len(t, result.Content, 1)
+		text, ok := result.Content[0].(*sdk.TextContent)
+		require.True(t, ok, "Expected TextContent")
+		assert.Equal(t, "tool execution failed", text.Text)
+	})
+
+	t.Run("nil error substitutes unknown error message", func(t *testing.T) {
+		result, second, returnedErr := NewErrorCallToolResult(nil)
+
+		require.NotNil(t, result)
+		assert.True(t, result.IsError)
+		assert.Nil(t, second)
+		require.Error(t, returnedErr)
+		assert.Equal(t, "unknown error", returnedErr.Error())
+
+		require.Len(t, result.Content, 1)
+		text, ok := result.Content[0].(*sdk.TextContent)
+		require.True(t, ok, "Expected TextContent")
+		assert.Equal(t, "unknown error", text.Text)
+	})
+
+	t.Run("error message with special characters is preserved", func(t *testing.T) {
+		inputErr := errors.New(`backend error: {"code":500,"message":"internal server error"}`)
+
+		result, _, returnedErr := NewErrorCallToolResult(inputErr)
+
+		require.NotNil(t, result)
+		assert.True(t, result.IsError)
+		assert.ErrorIs(t, returnedErr, inputErr)
+
+		require.Len(t, result.Content, 1)
+		text, ok := result.Content[0].(*sdk.TextContent)
+		require.True(t, ok)
+		assert.Equal(t, inputErr.Error(), text.Text)
+	})
+}
+
+// TestConvertToCallToolResult_MarshalError tests the error path when data cannot be marshaled.
+func TestConvertToCallToolResult_MarshalError(t *testing.T) {
+	// Channels cannot be marshaled to JSON; json.Marshal returns an error.
+	result, err := ConvertToCallToolResult(make(chan int))
+
+	assert.Error(t, err)
+	assert.Nil(t, result)
+	assert.Contains(t, err.Error(), "failed to marshal backend result")
+}