refactor: remove redundant config default fallbacks in server and launcher

[lpcox]

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):

payloadDir := config.DefaultPayloadDir
if cfg.Gateway != nil && cfg.Gateway.PayloadDir != "" {
    payloadDir = cfg.Gateway.PayloadDir
}
// ... repeated for PayloadPathPrefix, PayloadSizeThreshold

After (direct field access, 3 lines):

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):

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):

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

[Copilot]

Pull request overview

This PR removes duplicated “fallback default” logic in server/launcher consumers by introducing a config-level helper that guarantees cfg.Gateway is non-nil and defaulted, then switching call sites to rely on direct field access.

Changes:

• Added (*config.Config).EnsureGatewayDefaults() to centralize Gateway nil-safety + default application.
• Updated internal/server/unified.go and internal/launcher/launcher.go to call EnsureGatewayDefaults() and read cfg.Gateway.* directly.
• Adjusted launcher tests/comments to reflect that launcher.New now handles a nil Gateway.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
internal/config/config_core.go	Adds exported helper to ensure Gateway exists and applies gateway + feature defaults.
internal/server/unified.go	Removes local default fallbacks for payload config; relies on defaulted cfg.Gateway fields.
internal/launcher/launcher.go	Removes local startup-timeout fallback; relies on defaulted cfg.Gateway.StartupTimeout.
internal/launcher/launcher_test.go	Updates test setup/commentary for nil-gateway defaulting behavior.

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