fix: exposeAll strategy works with excludeExtraneousValues

[kyungseopk1m]

When strategy is explicitly set to 'exposeAll', preserve the keys collected from the source object instead of overwriting them with only decorator-registered exposed properties.

Fixes #740

Description

When using strategy: 'exposeAll' with excludeExtraneousValues: true, plainToInstance() returns {} instead of the expected properties. This happens because getKeys() unconditionally overwrites keys collected by exposeAll with exposedProperties, which is empty for classes without @Expose() decorators.

The fix checks this.options.strategy (user-provided) so it only activates when the user explicitly opts in — existing behavior for classes using @Expose() without an explicit strategy is unchanged.

Known limitation: With exposeAll, TypeScript properties without initializers or decorators are not discoverable at runtime, so excludeExtraneousValues cannot filter extraneous source properties in this combination. This is an inherent TypeScript limitation.

Checklist

• the pull request title describes what this PR does (not a vague title like Update index.md)

• the pull request targets the default branch of the repository (develop)

• the code follows the established code style of the repository

  • npm run prettier:check passes
  • npm run lint:check passes

• tests are added for the changes I made (if any source code was modified)

• documentation added or updated

• I have run the project locally and verified that there are no errors

  Fixes

  fixes fix: ExposeAll does not work with excludeExtraneousValues #740