fix(agent): guard switch_model when fallback chain is unset

[LeonSGP43]

Summary\n- normalize fallback state at the start of AIAgent.switch_model()\n- avoid crashing when lightweight callers or tests invoke switch_model() without the full init path\n- preserve existing fallback-pruning behavior when switching primary providers\n\n## Why\nRecent CI failures across authored PRs include tests/agent/test_minimax_provider.py::TestMinimaxSwitchModelCredentialGuard::test_switch_to_minimax_does_not_resolve_anthropic_token, which crashes with an AttributeError before the actual credential guard can be verified.\n\nThis patch keeps switch_model() robust for those direct-call paths and removes one shared test failure from the current PR backlog.\n\n## Verification\n- python3.12 direct repro of the failing MiniMax switch_model path now passes without _fallback_chain present\n- python3.12 direct repro of fallback-pruning compatibility still passes

[alt-glitch]

Likely duplicate of #14867 — same root cause: switch_model() missing _fallback_chain guard on partially-constructed agents. Also overlaps with merged #15134 which includes consolidated fallback fixes.

[teknium1]

Thanks for the contribution, @LeonSGP43! This fix was already merged into main via the consolidated fallback/retry PR #15134 (merged 2026-04-24T12:35:44Z), which explicitly credits your earlier PR #14867 as the source.

Evidence:

• run_agent.py line 2201 already contains fallback_chain = list(getattr(self, "_fallback_chain", []) or []) — the exact guard proposed here.
• PR fix(agent): consolidated fallback/retry correctness — SSL retry, cooldown, config context, auth fallback parity #15134 body includes: "run_agent.py (switch_model fallback state guard) — … From @LeonSGP43 (fix: default missing fallback state in switch_model #14867)."

This is an automated hermes-sweeper review. Closing as implemented on main.