fix(agentchat): prevent livelock in SelectorGroupChat fallback when allow_repeated_speaker=False

[1Ckpwee]

Summary

• Fix a livelock bug in SelectorGroupChat where the fallback after exhausting max_selector_attempts returns self._previous_speaker even when allow_repeated_speaker=False, causing an infinite loop (A speaks -> selector fails to pick non-A -> falls back to A -> repeat)
• When allow_repeated_speaker=False and selector attempts are exhausted, the fallback now picks a random participant that is NOT the previous speaker
• Updated existing test to explicitly set allow_repeated_speaker=True for the case that tests fallback-to-previous behavior
• Added new test test_selector_group_chat_no_livelock_on_fallback_when_repeated_speaker_disallowed to verify the fix

Fixes #7471

Test plan

• Existing test test_selector_group_chat_fall_back_to_previous_after_3_attempts continues to pass (now explicitly uses allow_repeated_speaker=True)
• New test verifies that when allow_repeated_speaker=False and the LLM exhausts all attempts by picking the previous speaker, the fallback selects a different participant
• Edge case: if only one participant exists, fallback still returns the previous speaker (no alternative available)

🤖 Generated with Claude Code

[1Ckpwee]

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