refactor(core): split the swagger dev feature pruning into its own module

[wangsijie]

Stack: based on #9774 → #9773. The feature, #9771, sits on top. Merge in that order.

Summary

utils/general.ts sat exactly at the 300-line max-lines limit, so there was no room for the response-header pruning the Account API step-up work needs. The dev-feature plumbing moves to utils/dev-feature.ts:

• devFeatureSchemaExtension, removeDevFeatureParameters, and removeDevFeatureSchemaProperties move as they are.
• removeDevFeatureResponseHeaders is new: it drops a response header marked with x-logto-dev-feature while the marker is still on it, and is called from assembleSwaggerDocument before removeDevFeatureSchemaProperties, which strips the marker from every record unconditionally.
• general.test.ts gains the two cases for the new pruning, and the moved imports and tests point at the new module.

Nothing in the repository marks a response header yet, so the served documents are unchanged and no changeset is needed. This is plumbing: the header it hides is added by the Account API step-up change.

Part of LOG-14124.

Testing

Unit tests: the new pruning cases run with dev features both disabled and enabled, and assert that the internal marker never reaches the document.

Checklist

• unit tests
• necessary TSDoc comments

[wangsijie]

Dropping this: it existed only to hide the new WWW-Authenticate response header from the served OpenAPI document, and the header is no longer described in the supplement at all. Its documentation will land with the release that makes the challenge unconditional, where the plumbing is not needed. The branch stays around in case we want the response-header pruning for something else later.