Closes#33175.
switch_model() in agent/agent_runtime_helpers.py mutated agent.model and
agent.provider before rebuilding the client, with no try/except to restore
them on failure. If the rebuild raised (bad API key, network error,
build_anthropic_client failure, etc.) the agent was left with the new
model+provider name paired with the OLD client — producing HTTP 400s like
"claude-sonnet-4-6 is not supported on openai-codex" on the next turn.
Callers in cli.py, gateway/run.py, and tui_gateway/server.py already catch
the exception and warn the user, but the warning was misleading because
the swap had partially succeeded; the agent's state was torn.
Snapshot every mutated field before the swap, wrap the swap+rebuild block
in try/except, and restore the snapshot on failure before re-raising so
the caller's warning surfaces.
Reported by @amirariff91. Tests cover both branches (chat_completions and
anthropic_messages) and the cross-branch case (anthropic -> openai).
