fix(agentchat): SelectorGroupChat fallback no longer returns excluded previous speaker

[alvinttang]

Summary

Fixes a livelock in SelectorGroupChat when allow_repeated_speaker=False: after the model exhausts max_selector_attempts, _select_speaker unconditionally returned self._previous_speaker — the very speaker that is supposed to be excluded — so if the model kept picking that speaker, the group chat could loop forever on the same agent.

Fixes #7471

Root cause

_select_speaker in _selector_group_chat.py has two fallback arms:

1. When all max_selector_attempts fail, reuse the previous speaker.
2. Or, if there is no previous speaker, use the first participant.

Arm (1) ran regardless of allow_repeated_speaker. When that flag is False, the caller has already filtered the previous speaker out of participants, so reusing them breaks the user-visible contract and livelocks if the model keeps selecting them.

Fix

• When allow_repeated_speaker=True, behaviour is unchanged.
• When allow_repeated_speaker=False, pick the first candidate in participants that is not self._previous_speaker, with participants[0] as a defensive default.
• The no-candidate_func path already pre-filters participants; the candidate_func path does not, so the same explicit skip protects both paths.
• Updated the max_selector_attempts docstring to reflect the new fallback behaviour.

Tests

python/packages/autogen-agentchat/tests/test_group_chat.py:

• test_selector_group_chat_fall_back_excludes_previous_when_disallowed — default allow_repeated_speaker=False path with three agents; verifies the fallback picks agent1 or agent3, never the repeated agent2.
• test_selector_group_chat_fall_back_excludes_previous_with_candidate_func — same scenario but driven via candidate_func, which does NOT pre-filter. Places agent2 first in the candidate list to exercise the worst case.
• test_selector_group_chat_fall_back_to_previous_after_3_attempts — the existing test was adjusted to set allow_repeated_speaker=True, because it was previously codifying the buggy default behaviour.

Locally on darwin/arm64 with uv:

• pytest tests/test_group_chat.py -k "fall_back_excludes_previous" → 4 passed, 87 deselected.
• pytest tests/test_group_chat.py -k "selector" → 32 passed.
• ruff check clean, pyright 0 errors.

Risk notes

Behavioural change is observable only when the selector model exhausts max_selector_attempts; users who relied on the prior "always fall back to previous speaker even when disallowed" behaviour need to opt into it with allow_repeated_speaker=True.