fix(agent): roll back switch_model() state when client rebuild fails

[teknium1]

Summary

switch_model() now rolls back to the pre-swap state when the client rebuild fails, so the agent isn't left with a new model/provider name paired with the old client.

Closes #33175 (reported by @amirariff91).

Root cause

agent/agent_runtime_helpers.py:switch_model() set agent.model and agent.provider at lines 1370-1371 before rebuilding the client at lines 1387-1438, with no try/except. An exception in the rebuild block (bad API key, network error, build_anthropic_client failure, MiniMax OAuth init failure, OpenAI client construction failure) left the agent with the new model+provider name but the OLD client. The next turn hit the mismatched combo and returned HTTP 400 (e.g. claude-sonnet-4-6 is not supported when using Codex with a ChatGPT account).

Callers in cli.py:7421, gateway/run.py:10261,10398, and tui_gateway/server.py:1164 already catch the exception and warn the user, but the warning was misleading — the swap had partially succeeded and the agent's state was torn.

Changes

• agent/agent_runtime_helpers.py — snapshot every field the swap+rebuild can mutate (model, provider, base_url, api_mode, api_key, client, _client_kwargs, _anthropic_client, _anthropic_api_key, _anthropic_base_url, _is_anthropic_oauth, _config_context_length) using a _MISSING sentinel so genuinely-unset attributes (bare __new__ agents in tests) aren't fabricated on rollback. Wrap the swap+rebuild in try/except, restore on failure, re-raise.
• tests/run_agent/test_switch_model_rollback.py — 4 new tests covering both branches and the cross-branch case.

Validation

	Before	After
switch_model failure leaves torn state	yes	no — rollback restores all fields
Targeted suite (model-switch tests)	n/a	131/131 passing across 12 files
New rollback regression tests	n/a	4/4 passing

Test scenarios covered:

1. OpenAI client rebuild raises → model/provider/base_url/api_mode/api_key/client/_client_kwargs all restored
2. build_anthropic_client raises → anthropic state (_anthropic_client, _anthropic_api_key, _anthropic_base_url) + core state restored
3. Cross-branch swap (anthropic → openai) with openai failure → anthropic client preserved, not nulled
4. Happy path still works — the try/except wrapper doesn't break successful switches

[github-actions]

🔎 Lint report: hermes/hermes-b93989a4 vs origin/main

ruff

Total: 0 on HEAD, 0 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 0 pre-existing issues carried over.

ty (type checker)

Total: 9495 on HEAD, 9494 on base (🆕 +1)

🆕 New issues (1):

Rule	Count
unresolved-import	1

First entries

tests/run_agent/test_switch_model_rollback.py:18: [unresolved-import] unresolved-import: Cannot resolve imported module `pytest`

✅ Fixed issues: none

Unchanged: 5013 pre-existing issues carried over.

Diagnostics are surfaced as warnings — this check never fails the build.