fix(config): skip native-loader compat warning when loader is explicit

[volcanicll]

Fixes #23267

Summary

The (!) Your Vite config uses features that are unsupported by configLoader: 'native' warning fired even when the user explicitly set --config-loader=bundle (or a bundler loader via API). Since an explicit choice means the native loader will never be used, the warning is noise in that case.

Fix

Make configLoader optional in loadConfigFromFile so an unset value can be distinguished from an explicit 'bundle', and only emit the compat warning on the default path:

• unset → defaults to 'bundle', warning fires (migration heads-up preserved)
• explicit 'bundle' / 'runner' → no warning
• explicit 'native' → bundler path not taken, no change

Test

• Existing 8 native-incompatibility warning tests updated to call through the default path
• Added regression test: explicit configLoader: 'bundle' produces no warnings
• All 109 tests in config.spec.ts pass; tsc --noEmit passes

[btea]

It looks like it's a duplicate of #23275.