Agent configuration edits already had an API path for replacing the full adapterConfig, but the edit form was still sending merge-style patches. That meant clearing the last environment variable serialized as undefined, the key disappeared from JSON, and the server merged the old env bindings back into the saved config.
Build adapter config save payloads as full replacement patches, strip undefined keys before send, and reuse the existing replaceAdapterConfig contract so explicit clears persist correctly. Add regression coverage for the cleared-env case and for adapter-type changes that still need to preserve adapter-agnostic fields.
Fixes#3179
