fix(agentchat): respect allow_repeated_speaker=False in selector fallback

[extrasmall0]

Fixes #7471

Problem

When allow_repeated_speaker=False and the model exhausts max_selector_attempts without picking a valid (non-previous) speaker, the fallback in _select_speaker returns self._previous_speaker:

if self._previous_speaker is not None:
    trace_logger.warning(...)
    return self._previous_speaker  # ← returns the excluded speaker

This creates a livelock: agent A speaks → selector tries to pick someone else → exhausts attempts → falls back to A → A speaks again → repeat.

Root Cause

The _select_speaker method receives a participants list that already has the previous speaker filtered out (done by the caller when allow_repeated_speaker=False). The fallback logic ignores this filtered list and reads directly from self._previous_speaker.

Fix

Add an early-exit branch before the existing fallback: when allow_repeated_speaker=False and there is a previous speaker, use random.choice(participants) (the already-filtered list) instead of returning the excluded speaker.

if self._previous_speaker is not None and not self._allow_repeated_speaker:
    # participants already excludes the previous speaker
    return random.choice(participants)

The existing fallback for allow_repeated_speaker=True (return previous speaker) is unchanged.

[extrasmall0]

Gentle ping — the fix is small and all CI checks are passing. Happy to add tests or make any adjustments if needed. Thanks for maintaining this!