[test-improver] Improve tests for mcp tool_result package (#4254)

# Test Improvements: `tool_result_test.go`

## File Analyzed

- **Test File**: `internal/mcp/tool_result_test.go`
- **Package**: `internal/mcp`
- **Lines Added**: +38

## Improvements Made

### 1. Increased Coverage

- ✅ Added test for `ConvertToCallToolResult` when the `content` field is
a non-array value (e.g. a JSON string) — triggers the `json.Unmarshal`
error fallback that wraps raw bytes as a text content item
- ✅ Added test documenting `ParseToolArguments` behavior with `null`
JSON input — shows it returns a `nil` map (not empty map) without error

- **`ConvertToCallToolResult` coverage**: 93.1% → **100%**
- **`ParseToolArguments` coverage**: maintained at 100%
- **`NewErrorCallToolResult` coverage**: maintained at 100%

### 2. Cleaner & More Stable Tests

- ✅ New tests follow the existing `t.Run()` subtest style in the file
(not table-driven, consistent with surrounding code)
- ✅ Use `require`/`assert` from testify for precise, idiomatic
assertions

## Test Execution

All `internal/mcp` tests pass:
```
ok  	github.com/github/gh-aw-mcpg/internal/mcp	0.744s	coverage: 82.0% of statements
github.com/github/gh-aw-mcpg/internal/mcp/tool_result.go:15:	ConvertToCallToolResult	100.0%
github.com/github/gh-aw-mcpg/internal/mcp/tool_result.go:131:	ParseToolArguments		100.0%
github.com/github/gh-aw-mcpg/internal/mcp/tool_result.go:148:	NewErrorCallToolResult	100.0%
```

*Note: `TestFetchAndFixSchema_NetworkError` in `internal/config` is a
pre-existing failure unrelated to this PR (network test making a real
HTTP request that returns 403 in CI).*

## Why These Changes?

`ConvertToCallToolResult` had an untested fallback path: when the
response JSON has a `content` field that is not an array (e.g.
`{"content": "some string"}`), the typed struct unmarshal fails and the
function falls back to wrapping the raw bytes as a text content item.
This path was completely untested.

The `ParseToolArguments` null-JSON test documents a subtle but important
behavior: passing `null` JSON yields a `nil` map (not an empty map),
which callers should be aware of.

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Author: lpcox

internal/mcp/tool_result_test.go

@@ -221,6 +221,29 @@ func TestConvertToCallToolResult(t *testing.T) {
 		require.NoError(t, err)
 		require.NotNil(t, result)
 	})
+
+	// This test exercises the fallback path in ConvertToCallToolResult where
+	// json.Unmarshal into the typed backendResult struct fails. This happens
+	// when the "content" field exists but holds a non-array JSON value (e.g. a
+	// string). The function should fall back to wrapping the raw bytes as text.
+	t.Run("content field is non-array value falls back to raw text wrap", func(t *testing.T) {
+		// Use a raw JSON map so we can provide a string-typed "content" field
+		// that passes the hasContentField check but fails the typed unmarshal.
+		input := map[string]interface{}{
+			"content": "this is a string, not an array",
+		}
+
+		result, err := ConvertToCallToolResult(input)
+
+		require.NoError(t, err)
+		require.NotNil(t, result)
+		assert.Len(t, result.Content, 1)
+		assert.False(t, result.IsError)
+
+		text, ok := result.Content[0].(*sdk.TextContent)
+		require.True(t, ok, "Expected TextContent fallback")
+		assert.Contains(t, text.Text, "this is a string, not an array")
+	})
 }
 
 // TestParseToolArguments tests extraction and unmarshaling of tool arguments.
@@ -317,6 +340,21 @@ func TestParseToolArguments(t *testing.T) {
 		require.NoError(t, err)
 		assert.Empty(t, args)
 	})
+
+	t.Run("null json arguments returns nil map without error", func(t *testing.T) {
+		// json.Unmarshal("null", &map) is valid and yields a nil map.
+		// The function returns (nil, nil) in this case.
+		req := &sdk.CallToolRequest{
+			Params: &sdk.CallToolParamsRaw{
+				Arguments: json.RawMessage(`null`),
+			},
+		}
+
+		args, err := ParseToolArguments(req)
+
+		require.NoError(t, err)
+		assert.Nil(t, args, "null JSON arguments should yield a nil map")
+	})
 }
 
 // TestNewErrorCallToolResult tests construction of error CallToolResult values.