refactor: remove redundant config default fallbacks in server and launcher (#3032)

## Summary

Removes duplicated config default logic identified in #2983 / #2984.
Consumer code in `unified.go` and `launcher.go` re-implemented config
field defaults that the config loading layer already guarantees.

## Changes

### New: `Config.EnsureGatewayDefaults()` (`config_core.go`)
Exported method that guarantees `cfg.Gateway` is non-nil and all
gateway-level + feature defaults are applied. This makes the config
loading contract explicit and safe for any consumer — including test
code that constructs configs manually without going through
`LoadFromFile`/`LoadFromStdin`.

### Simplified: `internal/server/unified.go`
**Before** (3 nil-guard + fallback blocks, 18 lines):
```go
payloadDir := config.DefaultPayloadDir
if cfg.Gateway != nil && cfg.Gateway.PayloadDir != "" {
    payloadDir = cfg.Gateway.PayloadDir
}
// ... repeated for PayloadPathPrefix, PayloadSizeThreshold
```

**After** (direct field access, 3 lines):
```go
cfg.EnsureGatewayDefaults()
payloadDir := cfg.Gateway.PayloadDir
payloadPathPrefix := cfg.Gateway.PayloadPathPrefix
payloadSizeThreshold := cfg.Gateway.PayloadSizeThreshold
```

### Simplified: `internal/launcher/launcher.go`
**Before** (nil-guard + fallback + branch logging, 8 lines):
```go
startupTimeout := time.Duration(config.DefaultStartupTimeout) * time.Second
if cfg.Gateway != nil && cfg.Gateway.StartupTimeout > 0 {
    startupTimeout = time.Duration(cfg.Gateway.StartupTimeout) * time.Second
}
```

**After** (direct access, 2 lines):
```go
cfg.EnsureGatewayDefaults()
startupTimeout := time.Duration(cfg.Gateway.StartupTimeout) * time.Second
```

### Updated: `launcher_test.go`
Updated test name and comment for `TestLauncher_TimeoutWithNilGateway`
to document that `EnsureGatewayDefaults` now handles the nil case.

## Why EnsureGatewayDefaults instead of just removing nil-guards?

100+ test files construct `config.Config{}` without setting `Gateway`.
Rather than touching all of them, `EnsureGatewayDefaults()` provides a
single defensive call that makes the contract explicit while keeping
existing tests working.

Closes #2983
Closes #2984

Author: lpcox

internal/config/config_core.go

@@ -198,6 +198,19 @@ func applyGatewayDefaults(cfg *GatewayConfig) {
 	}
 }
 
+// EnsureGatewayDefaults guarantees that cfg.Gateway is non-nil and that all
+// gateway-level fields have sensible defaults applied. This matches the
+// invariants enforced by the standard loaders (LoadFromFile, LoadFromStdin),
+// and can be used by callers that construct Config values manually (e.g. in
+// tests) to avoid nil-pointer panics and ensure consistent defaults.
+func (cfg *Config) EnsureGatewayDefaults() {
+	if cfg.Gateway == nil {
+		cfg.Gateway = &GatewayConfig{}
+	}
+	applyGatewayDefaults(cfg.Gateway)
+	applyDefaults(cfg)
+}
+
 // LoadFromFile loads configuration from a TOML file.
 //
 // This function uses the BurntSushi/toml v1.6.0+ parser with TOML 1.1 support,

internal/launcher/launcher.go

@@ -55,14 +55,13 @@ func New(ctx context.Context, cfg *config.Config) *Launcher {
 		log.Println("[LAUNCHER] Detected running inside a container")
 	}
 
-	// Get startup timeout from config, default to config.DefaultStartupTimeout seconds
-	startupTimeout := time.Duration(config.DefaultStartupTimeout) * time.Second
-	if cfg.Gateway != nil && cfg.Gateway.StartupTimeout > 0 {
-		startupTimeout = time.Duration(cfg.Gateway.StartupTimeout) * time.Second
-		logLauncher.Printf("Using configured startup timeout: %v", startupTimeout)
-	} else {
-		logLauncher.Printf("Using default startup timeout: %v", startupTimeout)
-	}
+	// Guarantee cfg.Gateway is non-nil with defaults applied.
+	// LoadFromFile/LoadFromStdin already ensure this, but tests may
+	// construct configs manually without going through the load path.
+	cfg.EnsureGatewayDefaults()
+
+	startupTimeout := time.Duration(cfg.Gateway.StartupTimeout) * time.Second
+	logLauncher.Printf("Using startup timeout: %v", startupTimeout)
 
 	// Initialize OIDC provider from environment if available
 	var oidcProvider *oidc.Provider

internal/launcher/launcher_test.go

@@ -527,6 +527,7 @@ func TestGetOrLaunchForSession_SessionReuse(t *testing.T) {
 	ctx := context.Background()
 	cfg := &config.Config{
 		Servers: map[string]*config.ServerConfig{},
+		Gateway: &config.GatewayConfig{StartupTimeout: config.DefaultStartupTimeout},
 	}
 	l := New(ctx, cfg)
 	defer l.Close()
@@ -606,7 +607,8 @@ func TestLauncher_StartupTimeout(t *testing.T) {
 }
 
 func TestLauncher_TimeoutWithNilGateway(t *testing.T) {
-	// Test that launcher uses default timeout when Gateway config is nil
+	// Test that launcher uses default timeout when Gateway config is nil.
+	// EnsureGatewayDefaults is called by launcher.New, so nil Gateway is safe.
 	ctx := context.Background()
 	cfg := &config.Config{
 		Servers: map[string]*config.ServerConfig{
@@ -615,7 +617,7 @@ func TestLauncher_TimeoutWithNilGateway(t *testing.T) {
 				URL:  "http://example.com",
 			},
 		},
-		Gateway: nil, // No gateway config
+		Gateway: nil, // No gateway config — EnsureGatewayDefaults fills defaults
 	}
 
 	l := New(ctx, cfg)

internal/server/unified.go

@@ -110,25 +110,14 @@ type UnifiedServer struct {
 // NewUnified creates a new unified MCP server
 func NewUnified(ctx context.Context, cfg *config.Config) (*UnifiedServer, error) {
 	logUnified.Printf("Creating new unified server: sequentialLaunch=%v, servers=%d", cfg.SequentialLaunch, len(cfg.Servers))
-	l := launcher.New(ctx, cfg)
-
-	// Get payload directory from config, with fallback to default
-	payloadDir := config.DefaultPayloadDir
-	if cfg.Gateway != nil && cfg.Gateway.PayloadDir != "" {
-		payloadDir = cfg.Gateway.PayloadDir
-	}
 
-	// Get payload path prefix from config (empty by default)
-	payloadPathPrefix := ""
-	if cfg.Gateway != nil && cfg.Gateway.PayloadPathPrefix != "" {
-		payloadPathPrefix = cfg.Gateway.PayloadPathPrefix
-	}
+	l := launcher.New(ctx, cfg)
 
-	// Get payload size threshold from config, with fallback to default
-	payloadSizeThreshold := config.DefaultPayloadSizeThreshold
-	if cfg.Gateway != nil && cfg.Gateway.PayloadSizeThreshold > 0 {
-		payloadSizeThreshold = cfg.Gateway.PayloadSizeThreshold
-	}
+	// Config loading guarantees cfg.Gateway is non-nil and all fields
+	// have defaults applied via applyGatewayDefaults/applyDefaults.
+	payloadDir := cfg.Gateway.PayloadDir
+	payloadPathPrefix := cfg.Gateway.PayloadPathPrefix
+	payloadSizeThreshold := cfg.Gateway.PayloadSizeThreshold
 	logUnified.Printf("Payload configuration: dir=%s, pathPrefix=%s, sizeThreshold=%d bytes (%.2f KB)",
 		payloadDir, payloadPathPrefix, payloadSizeThreshold, float64(payloadSizeThreshold)/1024)