agentHost: Simplify SSH relay reconnect with dispose-and-recreate pattern

[roblourens]

Refactor SSHConnection to use the same immutable dispose-and-recreate pattern as TunnelConnection instead of the more complex isActiveRelay()/replaceRelay() approach.

Problem

When a window reloads, we need to replace the WebSocket relay (for a fresh initialize handshake) while keeping the SSH session alive. The previous implementation mutated _relay inside a living SSHConnection and required every relay close handler to check isActiveRelay() to suppress stale close events. This was subtle and error-prone.

Solution

Adopt the pattern from TunnelConnection: connections are immutable, dispose-once objects. On reconnect, the old SSHConnection is disposed and a new one is created reusing the same SSH client.

Key changes

• Remove isActiveRelay() and replaceRelay() — SSHConnection._relay is now readonly
• Add detachSshClient() — prevents dispose() from ending the SSH session during ownership transfer; also removes event listeners from the shared SSH client
• Add removeListener to SSHClient interface — enables proper listener cleanup on detach
• Simplify relay close handlers — from if (conn?.isActiveRelay(relay)) { ... } to () => { conn?.dispose(); }
• Add error handling — try/catch in reconnect cleans up the detached SSH client if relay creation fails

Test coverage

18 new unit tests (33 total, up from 15):

• Event lifecycle ordering
• Relay message routing and multi-connection discrimination
• relaySend delivery and unknown-connectionId no-op
• Multi-host isolation (disconnect one doesn't affect other)
• Reconnect after disconnect creates fresh SSH client
• Progress events during connect
• SSH client close triggers connection disposal
• CLI install flow (skip/download)
• Connection key formats
• Reconnect preserves token/address
• Reconnect failure cleanup (SSH client not leaked)
• Listener cleanup across reconnects (close + error listener counts stable)

(Written by Copilot)

[Copilot]

Pull request overview

This PR refactors SSH relay reconnection in SSHRemoteAgentHostMainService to follow an immutable “dispose-and-recreate” connection model (matching the existing tunnel flow), avoiding in-place relay replacement and stale-close suppression logic.

Changes:

• Make SSHConnection immutable w.r.t. its relay and add detachSshClient() to transfer SSH client ownership during relay recreation.
• Update relay close handling to dispose the active SSHConnection (and suppress close effects from superseded connections via map-identity checks).
• Expand the SSH remote agent host unit tests to cover reconnect behavior, relay message routing, listener cleanup, and failure cleanup.

Show a summary per file

File	Description
src/vs/platform/agentHost/node/sshRemoteAgentHostService.ts	Implements dispose-and-recreate relay reconnect, adds SSH client listener detachment, and simplifies relay close handling.
src/vs/platform/agentHost/test/node/sshRemoteAgentHostService.test.ts	Adds/updates unit tests for reconnect semantics, relay routing, event ordering, and SSH client listener cleanup/leak prevention.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0