Add SSH remote agent host bootstrap (#304882)

* Add SSH remote agent host bootstrap

Adds a new ISSHRemoteAgentHostService that automates connecting to a
remote machine via SSH, installing the VS Code CLI, starting
'code agent-host', and forwarding the agent host port back through
the SSH tunnel.

- New service interface and types in common/sshRemoteAgentHost.ts
- Full implementation using ssh2 in electron-browser/ with dynamic
  imports to respect layering rules
- Multi-step quick input flow for SSH connection details integrated
  into the remote agent host picker
- 'Connect via SSH' command registered in contributions

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address PR review feedback

- Strip password/privateKeyPath from ISSHAgentHostConnection.config
  so secrets are not exposed to consumers after connect
- Redact connection tokens (?tkn=...) in all log output and error
  messages to prevent credential leakage
- Parse user@host:port format in SSH host input with proper validation
  for port range and missing components
- Guard onDidClose with a closed flag to prevent double-fire when
  dispose and SSH close/error events overlap

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* SSH working through main process

Co-authored-by: Copilot <copilot@github.com>

* Resolve ssh configs

Co-authored-by: Copilot <copilot@github.com>

* progress

* Granular connect progress, test fix

Co-authored-by: Copilot <copilot@github.com>

* Test, refactor

Co-authored-by: Copilot <copilot@github.com>

* Resolve comments

Co-authored-by: Copilot <copilot@github.com>

* Get rid of cpu-features

* Move to shared process

Co-authored-by: Copilot <copilot@github.com>

* fixes

Co-authored-by: Copilot <copilot@github.com>

* add ssh2 to remote/package.json

* Cleanup and fixes

Co-authored-by: Copilot <copilot@github.com>

* fix

Co-authored-by: Copilot <copilot@github.com>

* fix

Co-authored-by: Copilot <copilot@github.com>

* resolve comments

Co-authored-by: Copilot <copilot@github.com>

* comments

Co-authored-by: Copilot <copilot@github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Raymond Zhao <7199958+rzhao271@users.noreply.github.com>
Co-authored-by: Copilot <copilot@github.com>

remote/package.json

@@ -42,6 +42,7 @@
     "vscode-regexpp": "^3.1.0",
     "vscode-textmate": "^9.3.2",
     "ws": "^8.19.0",
+    "ssh2": "^1.16.0",
     "yauzl": "^3.0.0",
     "yazl": "^2.4.3"
   },
@@ -49,6 +50,9 @@
     "node-gyp-build": "4.8.1",
     "kerberos@2.1.1": {
       "node-addon-api": "7.1.0"
+    },
+    "ssh2": {
+      "cpu-features": "0.0.0"
     }
   }
 }