Honor /compact summary when auto-summarization is disabled (#318132)

* Honor /compact summary when auto-summarization is disabled

Fixes #311503. With chat.summarizeAgentConversationHistory.enabled=false the
agent renders history via AgentConversationHistory, which replayed every turn's
full tool-call history and ignored round.summary. After a manual /compact, the
next turn rebuilt the entire pre-compaction context and the context window
immediately refilled.

AgentConversationHistory now finds the most recent summarized round (populated
by normalizeSummariesOnRounds), drops the history it supersedes, emits the
<conversation-summary> tag in place of that turn's user message, and replays
only the rounds after the summary. This applies an existing summary but never
triggers new summarization, which stays gated behind the setting. The no-summary
path is unchanged.

* Read summary rounds from a single source for index consistency