Fix reasoning text rendering when restoring agent host sessions

[roblourens]

When re-opening an agent host session, thinking text rendered all bunched up at the top of the response instead of being properly interspersed with tool calls.

Root cause

The SDK's session.getMessages() history bundles reasoning into each assistant.message as reasoningText, rather than emitting it as a separate event. mapSessionEvents already forwarded reasoningText onto IAgentMessageEvent, but _buildTurnsFromMessages only read content and toolRequests and dropped reasoning entirely. Whatever thinking text users were seeing on resume came from a different path that did not honor stream order, hence the bunching.

Fix

When handling an assistant.message in _buildTurnsFromMessages (and in _buildSubagentTurns), push a Reasoning response part from msg.reasoningText immediately before the Markdown part. Tool calls then naturally interleave because they arrive as separate tool_start / tool_complete events between messages.

This matches the shape the Copilot EH CLI uses for history replay (reasoning + text parts pulled off a single assistant.message).

Added regression tests in agentService.test.ts asserting the response parts come back in the order: reasoning → markdown → tool call → reasoning → markdown.

Also tidied up by introducing a SessionHistoryEvent type alias for the union of events getSessionMessages returns.

(Written by Copilot)

[Copilot]

Pull request overview

Fixes session restore rendering so assistant “reasoning/thinking” text replays in the correct stream order relative to markdown and tool calls when restoring agent host sessions.

Changes:

• Introduces SessionHistoryEvent as the canonical event union for IAgent.getSessionMessages() history reconstruction.
• Updates turn reconstruction to insert ResponsePartKind.Reasoning from IAgentMessageEvent.reasoningText immediately before the markdown part (including subagent turn reconstruction).
• Adds a regression test asserting reasoning → markdown → tool call ordering on resume.

Show a summary per file

File	Description
src/vs/platform/agentHost/common/agentService.ts	Adds SessionHistoryEvent and updates IAgent.getSessionMessages() return type.
src/vs/platform/agentHost/node/agentService.ts	Replays reasoningText into ResponsePartKind.Reasoning during history-to-turn reconstruction (parent + subagent).
src/vs/platform/agentHost/node/copilot/mapSessionEvents.ts	Uses SessionHistoryEvent for mapped history event arrays.
src/vs/platform/agentHost/node/copilot/copilotAgentSession.ts	Updates getMessages() to return SessionHistoryEvent[].
src/vs/platform/agentHost/node/copilot/copilotAgent.ts	Updates session message retrieval types to SessionHistoryEvent[].
src/vs/platform/agentHost/test/node/mockAgent.ts	Updates mock agent history typing to SessionHistoryEvent[] (note: one implementation still needs updating).
src/vs/platform/agentHost/test/node/agentService.test.ts	Adds regression coverage for reasoning/markdown/tool-call ordering on restore.

Copilot's findings

• Files reviewed: 7/7 changed files
• Comments generated: 1