[test-improver] Improve tests for logger/rpc_formatter (#4332)

## File Analyzed

- **Test File**: `internal/logger/rpc_logger_test.go`
- **Package**: `internal/logger`
- **Lines Added**: +94

## Improvements Made

### 1. Increased Coverage for `formatRPCMessage`

Added 3 new table-driven test cases covering untested branches in
`rpc_formatter.go`:

- ✅ **Empty `ServerID`** — verifies no server/direction prefix is added
when `serverID == ""`; only the payload size and payload content appear
- ✅ **Empty `Payload`** — verifies the payload string is omitted from
output when payload is empty
- ✅ **Error with no payload** — verifies an error-only response (no
payload) formats correctly with `err:` prefix

### 2. Increased Coverage for `formatRPCMessageMarkdown`

Added 3 new table-driven test cases covering untested branches:

- ✅ **Empty `ServerID`** — verifies no bold server prefix
(`**server**→`) is produced
- ✅ **Empty `Payload`** — verifies no `\`\`\`json` code block is emitted
when payload is empty
- ✅ **`tools/call` with non-string `name` in params** — verifies the
tool-name extraction guard (checking `toolName, ok :=
params["name"].(string)`) correctly skips appending a backtick-wrapped
name when the value is not a string

### 3. Increased Coverage for `formatJSONWithoutFields`

Added 2 new table-driven test cases:

- ✅ **Nil `fieldsToRemove` slice** — verifies all fields are preserved
when the removal list is nil
- ✅ **Non-existent field removal is a no-op** — verifies the function
handles attempts to remove fields that aren't in the JSON gracefully

## Test Execution

All tests pass:

```
=== RUN   TestFormatRPCMessage
--- PASS: TestFormatRPCMessage (0.00s)
    --- PASS: TestFormatRPCMessage/outbound_request
    --- PASS: TestFormatRPCMessage/inbound_response_with_error
    --- PASS: TestFormatRPCMessage/client_request
    --- PASS: TestFormatRPCMessage/empty_server_ID_omits_server_prefix
    --- PASS: TestFormatRPCMessage/empty_payload_omits_payload_from_output
    --- PASS: TestFormatRPCMessage/response_with_error_and_no_payload
=== RUN   TestFormatRPCMessageMarkdown
--- PASS: TestFormatRPCMessageMarkdown (0.00s)
    ...10 subtests all PASS...
=== RUN   TestFormatJSONWithoutFields
--- PASS: TestFormatJSONWithoutFields (0.00s)
    ...8 subtests all PASS...
ok  	github.com/github/gh-aw-mcpg/internal/logger	0.006s
```

## Why These Changes?

`rpc_formatter.go` contains the core text/markdown formatting logic for
all RPC message logging. The existing tests covered the "happy path"
scenarios (non-empty server, non-empty payload, valid JSON). However,
several defensive branches in the code — guarding against empty
`ServerID`, empty `Payload`, and non-string type assertions — had zero
test coverage. These new tests exercise those branches directly,
documenting the expected behavior and guarding against regressions.

---
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Author: lpcox

internal/logger/rpc_logger_test.go

@@ -55,6 +55,42 @@ func TestFormatRPCMessage(t *testing.T) {
 			},
 			want: []string{"client←tools/call", "200b"},
 		},
+		{
+			name: "empty server ID omits server prefix",
+			info: &RPCMessageInfo{
+				Direction:   RPCDirectionOutbound,
+				MessageType: RPCMessageRequest,
+				ServerID:    "",
+				Method:      "tools/list",
+				PayloadSize: 30,
+				Payload:     `{"method":"tools/list"}`,
+			},
+			want: []string{"30b", `{"method":"tools/list"}`},
+		},
+		{
+			name: "empty payload omits payload from output",
+			info: &RPCMessageInfo{
+				Direction:   RPCDirectionOutbound,
+				MessageType: RPCMessageRequest,
+				ServerID:    "github",
+				Method:      "tools/list",
+				PayloadSize: 0,
+				Payload:     "",
+			},
+			want: []string{"github→tools/list", "0b"},
+		},
+		{
+			name: "response with error and no payload",
+			info: &RPCMessageInfo{
+				Direction:   RPCDirectionInbound,
+				MessageType: RPCMessageResponse,
+				ServerID:    "backend",
+				PayloadSize: 0,
+				Payload:     "",
+				Error:       "connection reset",
+			},
+			want: []string{"backend←resp", "0b", "err:connection reset"},
+		},
 	}
 
 	for _, tt := range tests {
@@ -177,6 +213,46 @@ func TestFormatRPCMessageMarkdown(t *testing.T) {
 			want:    []string{"**github**→`tools/call`", "```json", `"params"`},
 			notWant: []string{`"jsonrpc"`, `"method"`},
 		},
+		{
+			name: "tools/call with non-string name in params does not append tool name",
+			info: &RPCMessageInfo{
+				Direction:   RPCDirectionOutbound,
+				MessageType: RPCMessageRequest,
+				ServerID:    "github",
+				Method:      "tools/call",
+				PayloadSize: 60,
+				Payload:     `{"jsonrpc":"2.0","method":"tools/call","params":{"name":42,"arguments":{}}}`,
+			},
+			// name is not a string so tool name extraction should not add backtick-wrapped name
+			want:    []string{"**github**→`tools/call`"},
+			notWant: []string{"**github**→`tools/call` `"},
+		},
+		{
+			name: "empty server ID produces no server prefix",
+			info: &RPCMessageInfo{
+				Direction:   RPCDirectionOutbound,
+				MessageType: RPCMessageRequest,
+				ServerID:    "",
+				Method:      "tools/list",
+				PayloadSize: 40,
+				Payload:     `{"jsonrpc":"2.0","method":"tools/list","params":{"key":"val"}}`,
+			},
+			want:    []string{"```json", `"params"`},
+			notWant: []string{"**"},
+		},
+		{
+			name: "empty payload produces no code block",
+			info: &RPCMessageInfo{
+				Direction:   RPCDirectionOutbound,
+				MessageType: RPCMessageRequest,
+				ServerID:    "github",
+				Method:      "tools/list",
+				PayloadSize: 0,
+				Payload:     "",
+			},
+			want:    []string{"**github**→`tools/list`"},
+			notWant: []string{"```json"},
+		},
 	}
 
 	for _, tt := range tests {
@@ -258,6 +334,24 @@ func TestFormatJSONWithoutFields(t *testing.T) {
 			wantValid:      true,
 			wantEmpty:      false,
 		},
+		{
+			name:           "nil fields to remove leaves all fields",
+			input:          `{"jsonrpc":"2.0","method":"tools/list","id":1}`,
+			fieldsToRemove: nil,
+			wantContains:   []string{`"jsonrpc"`, `"method"`, `"id"`},
+			wantNotContain: []string{},
+			wantValid:      true,
+			wantEmpty:      false,
+		},
+		{
+			name:           "removing non-existent field is a no-op",
+			input:          `{"id":1,"result":{}}`,
+			fieldsToRemove: []string{"jsonrpc", "method"},
+			wantContains:   []string{`"id"`, `"result"`},
+			wantNotContain: []string{`"jsonrpc"`, `"method"`},
+			wantValid:      true,
+			wantEmpty:      false,
+		},
 	}
 
 	for _, tt := range tests {