run_full_subagent replaced sub_chat.client by direct assignment after
construction, leaving ConversationCompressor (bound to the original
client in DeepSeekChat.__init__) and the api_key/name snapshots
pointing at the default model — compression requests silently kept
using the default endpoint/model while the dialogue itself used the
requested one.
DeepSeekChat gains set_client(): swaps self.client and refreshes
api_key, name, and compressor.llm_client in one place. run_full_subagent
now routes model overrides through it.
Tests: new TestM1SetClient (2 cases: full reference refresh on a real
DeepSeekChat instance, and run_full_subagent calling set_client with
the new client); full suite 295 passed.