Add debug logging to config_core.go LoadFromFile and applyGatewayDefaults

- Log undecoded TOML key count before unknown-field validation
- Log server count before stdio containerization validation
- Log server count before auth configuration validation
- Log when opentelemetry config is merged into tracing
- Log applied gateway defaults (port, timeouts, keepalive interval)

These log calls use the existing logConfig logger and are activated
via DEBUG=config:* or DEBUG=* to help diagnose configuration loading
issues at startup.

Co-authored-by: Copilot <[email protected]>

Author: github-actions[bot]

internal/config/config_core.go

@@ -260,6 +260,8 @@ func applyGatewayDefaults(cfg *GatewayConfig) {
 	if cfg.KeepaliveInterval == 0 {
 		cfg.KeepaliveInterval = DefaultKeepaliveInterval
 	}
+	logConfig.Printf("Applied gateway defaults: port=%d, startupTimeout=%d, toolTimeout=%d, keepaliveInterval=%d",
+		cfg.Port, cfg.StartupTimeout, cfg.ToolTimeout, cfg.KeepaliveInterval)
 }
 
 // EnsureGatewayDefaults guarantees that cfg.Gateway is non-nil and that all
@@ -350,6 +352,7 @@ func LoadFromFile(path string) (*Config, error) {
 	// Note: map[string]interface{} fields (guard_policies, guards.*.config) are
 	// intentionally flexible and their nested keys are exempt from this check.
 	undecoded := md.Undecoded()
+	logConfig.Printf("Checking %d undecoded TOML keys against allowed fields", len(undecoded))
 	var unknownKeys []toml.Key
 	for _, key := range undecoded {
 		if !isDynamicTOMLPath(key) {
@@ -370,13 +373,15 @@ func LoadFromFile(path string) (*Config, error) {
 	}
 
 	// Validate TOML stdio servers use Docker for containerization (Spec Section 3.2.1)
+	logConfig.Printf("Validating stdio server containerization requirements for %d servers", len(cfg.Servers))
 	if err := validateTOMLStdioContainerization(cfg.Servers); err != nil {
 		return nil, err
 	}
 
 	// Validate auth configs (e.g. fail-fast for missing OIDC env vars).
 	// This ensures parity with the JSON stdin path which calls validateServerAuth
 	// via convertStdinServerConfig → validateServerConfigWithCustomSchemas.
+	logConfig.Printf("Validating auth configuration for %d servers", len(cfg.Servers))
 	for name, serverCfg := range cfg.Servers {
 		jsonPath := fmt.Sprintf("servers.%s", name)
 		if err := validateServerAuth(serverCfg.Auth, serverCfg.Type, name, jsonPath); err != nil {
@@ -397,6 +402,7 @@ func LoadFromFile(path string) (*Config, error) {
 	// Merge opentelemetry key into tracing when present (spec §4.1.3.6).
 	// opentelemetry takes precedence over the legacy tracing key.
 	if cfg.Gateway.Opentelemetry != nil {
+		logConfig.Printf("opentelemetry section found: merging into tracing config, endpoint=%s", cfg.Gateway.Opentelemetry.Endpoint)
 		cfg.Gateway.Tracing = cfg.Gateway.Opentelemetry
 		cfg.Gateway.Opentelemetry = nil
 		// Expand ${VAR} expressions in tracing fields before validation.