config: make all settings experiment-aware by default (#300437)

* fix: improve experiment handling in configuration defaults

* refactor: simplify experiment property structure in configuration schemas

* Copilot CLI session f7a6aaaa-477b-497b-bfe8-9a59c2c61ed8 changes

* refactor: remove 'experimentMode: auto' from configuration schemas

* feat: export ConfigurationDefaultOverridesContribution class and add tests for configuration overrides

* fix: update experimentMode assignment logic in configuration properties

* fix: ensure experimentMode is always set to 'startup' in property configuration