fix(server): sort default toolset alphabetically for stable ordering (#3539)

## Summary

This PR resolves issue #3463 by sorting the tools in the default toolset
alphabetically during server initialization.

Currently, the default toolset (`cfg.ToolsetConfigs[""]`) is populated
by iterating over the `toolsMap`. Because map iteration order in Go is
randomized, tools were being returned in an arbitrary order across
server restarts. This broke client-side optimizations like Anthropic
Prompt Caching that rely on a stable tool order.

By sorting `allToolNames` with `slices.Sort()` prior to populating the
default toolset, we guarantee stable alphabetical ordering while leaving
custom toolsets (which are manually ordered in user configs) untouched.

## Testing
- Added unit test `TestDefaultToolsetIsAlphabeticallySorted` in
`internal/server/server_test.go` to explicitly verify that out-of-order
tools get sorted correctly in the default toolset.
- Verified that all existing server unit tests pass.

Fixes #3463

---------

Co-authored-by: Yuan Teoh <45984206+Yuan325@users.noreply.github.com>

Author: vismaytiwari

internal/server/server.go

@@ -335,6 +335,7 @@ func initializeToolsets(ctx context.Context, cfg ServerConfig, toolsMap map[stri
 	for name := range toolsMap {
 		allToolNames = append(allToolNames, name)
 	}
+	slices.Sort(allToolNames)
 	if cfg.ToolsetConfigs == nil {
 		cfg.ToolsetConfigs = make(ToolsetConfigs)
 	}

internal/server/server_test.go

@@ -1449,3 +1449,39 @@ func TestMCPAuthEnableAPIClash(t *testing.T) {
 		t.Errorf("unexpected error message: %v", err)
 	}
 }
+
+func TestDefaultToolsetIsAlphabeticallySorted(t *testing.T) {
+	ctx, err := testutils.ContextWithNewLogger()
+	if err != nil {
+		t.Fatalf("error setting up logger: %s", err)
+	}
+	instrumentation, err := telemetry.CreateTelemetryInstrumentation("0.0.0")
+	if err != nil {
+		t.Fatalf("unexpected error: %s", err)
+	}
+	ctx = util.WithInstrumentation(ctx, instrumentation)
+
+	cfg := server.ServerConfig{
+		Version: "0.0.0",
+		ToolConfigs: server.ToolConfigs{
+			"zoo":    offlineToolConfig{name: "zoo"},
+			"apple":  offlineToolConfig{name: "apple"},
+			"banana": offlineToolConfig{name: "banana"},
+		},
+	}
+
+	_, toolsetsMap, err := server.InitializeOfflineConfigs(ctx, cfg)
+	if err != nil {
+		t.Fatalf("InitializeOfflineConfigs returned error: %s", err)
+	}
+
+	defaultToolset, ok := toolsetsMap[""]
+	if !ok {
+		t.Fatal("expected default toolset to be present")
+	}
+
+	expectedOrder := []string{"apple", "banana", "zoo"}
+	if diff := cmp.Diff(expectedOrder, defaultToolset.ToolNames); diff != "" {
+		t.Errorf("default toolset ToolNames mismatch (-want +got):\n%s", diff)
+	}
+}