fix(acp): serialize session updates to prevent out-of-order messages

[LydiaCai1203]

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Type of change

• Bug fix

What does this PR do?

Problem: ACP session updates (sessionUpdate calls) can arrive at the client out of order because they are fired
concurrently from different event handlers. This causes garbled streaming output — for example, tool results
appearing before the tool call, or text chunks arriving in the wrong sequence. Additionally, there's a race
condition where SSE delta events haven't all arrived by the time end_turn fires, causing the final assistant message
to be truncated.

Fix:

1. Serialized session updates — All sessionUpdate calls are now routed through a single promise chain (sendUpdate /
   sendChain), guaranteeing they are delivered in the order they were enqueued. Individual .catch() handlers are
   consolidated into the chain.
2. Post-prompt reconciliation — After prompt() returns, reconcileAssistantMessage() fetches the complete assistant
   message from the API and delivers any text/reasoning content that the SSE delta events missed (comparing characters
   sent via deltas vs the final message length). A reconciledParts set prevents the delta handler from sending
   duplicates for parts already reconciled.
3. question.asked event handling — Adds a new event handler that maps AskUserQuestion tool calls to the ACP
   permission flow, with support for multi-question/multi-select responses and "Other" custom text answers.
4. Minor: adds debug logging for session processor stop reasons; updates the question tool to a two-step async
   pattern; adds missing question method to test context.

How did you verify your code works?

• Tested ACP streaming locally — verified message chunks arrive in correct order
• Verified truncated messages no longer occur after reconciliation
• Tested question flow end-to-end through ACP permission UI

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