Support session reference attachment for agent host copilot

[anthonykim1]

Part of: #315193 (comment)

• Include Agent Host Copilot sessions in session-reference attachment picker and #session: completions.
• Convert sessionReference variables into Agent Host Simple attachments instead of dropping them.
• Add session-reference attachment metadata so replay/resume restores the Sessions chip as kind: 'sessionReference'.
• Include bounded referenced-session transcript text in the attachment modelRepresentation when available, with debug-event fallback for sessions whose transcript is exposed that way.
• Reuse the shared Agent Host utility-context character budget when truncating attached session transcript text.
• Add focused coverage for outgoing conversion, replay restore, transcript hydration, debug fallback, and malformed session-reference values.

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Inspirations from:

• sessionReference variable shape comes from the Sessions picker attachment entry in chatContext.ts.
• Session ID derivation uses chatSessionResourceToId.
• Extension-host Copilot represents session references with session id plus URI target in sessionReferenceAttachmentAttrs.
• Transcript-size budgeting follows the existing Agent Host title-context budget from MAX_TITLE_CONTEXT_CHARS.
• #session: completion provider filtering is kept aligned with the session picker behavior in chatInputCompletions.ts.

[Copilot]

Pull request overview

Enables attaching session references for the agent-host Copilot flow by round-tripping session-reference variables through agent-host “simple” attachments, and enriching those attachments with referenced session transcript content when available.

Changes:

• Add a session-reference attachment format (displayKind + metadata) and restoration helper.
• Update agent-host variable→attachment conversion to hydrate referenced session transcripts via IChatSessionsService, with a debug-events fallback via IChatDebugService.
• Expand session picker filtering to include agent-host Copilot sessions and add targeted tests for restore/convert paths.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/test/browser/agentSessions/agentHostChatContribution.test.ts	Adds tests for restoring session-reference attachments and for converting sessionReference variables into agent-host simple attachments (including transcript hydration scenarios).
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/stateToProgressAdapter.ts	Restores session-reference variable entries from simple attachments during state→variable reconstruction.
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionReferenceAttachment.ts	New helper module defining the session-reference attachment shape, metadata, transcript rendering helpers, and restoration logic.
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionHandler.ts	Hydrates and serializes session-reference variables into attachments, including transcript embedding from session history or debug events.
src/vs/workbench/contrib/chat/browser/actions/chatContext.ts	Includes AgentHostCopilot in the session provider filter for the session reference picker.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 3
• Review effort level: Low

[anthonykim1]

@roblourens @connor4312 Copilot is having me add ChatDebugService to cover the scenario:

• Type some message and send + get response from session A
• Try to attach session from session A in session B.
• In ideal scenario, it would know the content, but without this fallback to ChatDebugService way, this "relatively fresh session" scenario leads to:

I don't really feel great adding both ChatSessionService and ChatDebugService, but it seems like ChatSessionService alone in the above scenario is not sufficient.

Could I get some wisdom