[test-improver] Improve tests for server/system_tools package (#5715)

# Test Improvements: system_tools_test.go

## File Analyzed

- **Test File**: `internal/server/system_tools_test.go`
- **Package**: `internal/server`

## Improvements Made

### 1. Fixed Bound Asserter Scope in Subtests

Three table-driven test functions created a `assert := assert.New(t)`
bound asserter against the **outer** test's `*testing.T`, then used it
inside `t.Run(...)` subtests where the inner `t` shadows the outer one.

**Problem:** When an assertion fails inside a subtest, the failure is
attributed to the *parent* test rather than the failing subtest. This
makes test output misleading and debugging harder — you see `--- FAIL:
TestHandleRequest_ToolsCall_InvalidJSON` instead of the specific subcase
like `--- FAIL: TestHandleRequest_ToolsCall_InvalidJSON/null_params`.

**Fix:** Moved `assert := assert.New(t)` inside each `t.Run` closure so
the bound asserter is correctly scoped to the subtest's `t`.

Affected functions:
- `TestHandleRequest_ToolsCall_InvalidJSON`
- `TestHandleRequest_ToolsCall_UnknownTool`
- `TestHandleRequest_UnsupportedMethod`

**Before:**
```go
func TestHandleRequest_ToolsCall_InvalidJSON(t *testing.T) {
    assert := assert.New(t)  // bound to outer t
    // ...
    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {  // inner t shadows outer t
            result, err := server.HandleRequest(...)
            assert.Error(err, ...)  // uses outer t — wrong!
        })
    }
}
```

**After:**
```go
func TestHandleRequest_ToolsCall_InvalidJSON(t *testing.T) {
    // ...
    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            assert := assert.New(t)  // bound to subtest t — correct
            result, err := server.HandleRequest(...)
            assert.Error(err, ...)
        })
    }
}
```

## Test Execution

All affected tests pass and failures are now correctly attributed to
their subtests:

```
--- PASS: TestHandleRequest_ToolsCall_InvalidJSON (0.00s)
    --- PASS: TestHandleRequest_ToolsCall_InvalidJSON/invalid_JSON (0.00s)
    --- PASS: TestHandleRequest_ToolsCall_InvalidJSON/missing_name_field (0.00s)
    --- PASS: TestHandleRequest_ToolsCall_InvalidJSON/null_params (0.00s)
    --- PASS: TestHandleRequest_ToolsCall_InvalidJSON/empty_object (0.00s)
    --- PASS: TestHandleRequest_ToolsCall_InvalidJSON/array_instead_of_object (0.00s)
    --- PASS: TestHandleRequest_ToolsCall_InvalidJSON/string_instead_of_object (0.00s)
--- PASS: TestHandleRequest_ToolsCall_UnknownTool (0.00s)
    --- PASS: TestHandleRequest_ToolsCall_UnknownTool/unknown_tool (0.00s)
    --- PASS: TestHandleRequest_ToolsCall_UnknownTool/misspelled_tool (0.00s)
    --- PASS: TestHandleRequest_ToolsCall_UnknownTool/case_sensitive_tool_name (0.00s)
    --- PASS: TestHandleRequest_ToolsCall_UnknownTool/empty_tool_name (0.00s)
--- PASS: TestHandleRequest_UnsupportedMethod (0.00s)
    --- PASS: TestHandleRequest_UnsupportedMethod/resources/list (0.00s)
    --- PASS: TestHandleRequest_UnsupportedMethod/prompts/list (0.00s)
    --- PASS: TestHandleRequest_UnsupportedMethod/empty_method (0.00s)
    --- PASS: TestHandleRequest_UnsupportedMethod/invalid_method (0.00s)
    --- PASS: TestHandleRequest_UnsupportedMethod/case_sensitive_method (0.00s)
```

## Why These Changes?

This is a subtle but real correctness issue with testify's bound
asserter pattern. The `assert.New(t)` call captures the provided `t` at
construction time. When used inside a `t.Run` closure where the inner
function parameter also happens to be named `t`, the *outer* bound
asserter is still referencing the *outer* test context. Test failures
are silently misattributed to the parent test, making subtest-level
debugging impossible.

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Author: lpcox

internal/server/system_tools_test.go

@@ -261,8 +261,6 @@ func TestHandleRequest_ToolsCall_ListServers(t *testing.T) {
 }
 
 func TestHandleRequest_ToolsCall_InvalidJSON(t *testing.T) {
-	assert := assert.New(t)
-
 	server := NewSysServer([]string{"github"})
 
 	tests := []struct {
@@ -297,6 +295,7 @@ func TestHandleRequest_ToolsCall_InvalidJSON(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
+			assert := assert.New(t)
 			result, err := server.HandleRequest("tools/call", tt.params)
 
 			assert.Error(err, "Should return error for invalid params")
@@ -307,8 +306,6 @@ func TestHandleRequest_ToolsCall_InvalidJSON(t *testing.T) {
 }
 
 func TestHandleRequest_ToolsCall_UnknownTool(t *testing.T) {
-	assert := assert.New(t)
-
 	server := NewSysServer([]string{"github"})
 
 	tests := []struct {
@@ -335,6 +332,7 @@ func TestHandleRequest_ToolsCall_UnknownTool(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
+			assert := assert.New(t)
 			params := json.RawMessage(`{
 "name": "` + tt.toolName + `",
 "arguments": {}
@@ -355,8 +353,6 @@ func TestHandleRequest_ToolsCall_UnknownTool(t *testing.T) {
 }
 
 func TestHandleRequest_UnsupportedMethod(t *testing.T) {
-	assert := assert.New(t)
-
 	server := NewSysServer([]string{"github"})
 
 	tests := []struct {
@@ -387,6 +383,7 @@ func TestHandleRequest_UnsupportedMethod(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
+			assert := assert.New(t)
 			result, err := server.HandleRequest(tt.method, nil)
 
 			assert.Error(err, "Should return error for unsupported method")