…ted agents

## Problem

`AIAgent.switch_model()` unconditionally reads ``self._fallback_chain``
when pruning the fallback chain after a provider swap (run_agent.py:2041).

If an agent is partially constructed — e.g. tests that use
``AIAgent.__new__(AIAgent)`` with ``__init__`` mocked, to isolate
``switch_model`` behavior — ``_fallback_chain`` may not exist yet,
causing:

    AttributeError: 'AIAgent' object has no attribute '_fallback_chain'

Reproduction: `pytest tests/agent/test_minimax_provider.py::\
TestMinimaxSwitchModelCredentialGuard::\
test_switch_to_minimax_does_not_resolve_anthropic_token`

Without this fix, the test fails with the above AttributeError at
run_agent.py:2042 because the test patches ``AIAgent.__init__`` to
`return_value=None` and never sets ``_fallback_chain``.

## Fix

Replace the direct attribute read with ``getattr(self, "_fallback_chain",
[]) or []``. This preserves the existing behaviour for fully-initialized
agents (the attribute is always a list in that case) while keeping
partially-constructed test fixtures working.

## Changes

- `run_agent.py:2041-2044`
  - Add ``existing_chain = getattr(self, "_fallback_chain", []) or []``
    before the list-comprehension.
  - Switch the comprehension to iterate ``existing_chain``.
  - 3-line change, no behavioural difference for production code paths.

## Testing

Verified the test failure reproduces without the fix and passes with it:

  # Without fix:
  AttributeError: 'AIAgent' object has no attribute '_fallback_chain'
  FAILED tests/agent/test_minimax_provider.py::\
    TestMinimaxSwitchModelCredentialGuard::\
    test_switch_to_minimax_does_not_resolve_anthropic_token

  # With fix:
  PASSED tests/agent/test_minimax_provider.py -q
  # 41 passed in 4.32s

## Risk

Minimal. `getattr(obj, name, default)` is idiomatic Python for defensive
attribute access and has zero runtime cost when the attribute exists.
The `or []` guards the (extremely unlikely) case where someone manually
set the attribute to None.

Signed-off-by: Andre Kurait <andrekurait@gmail.com>