Set TERM_PROGRAM for Agent Host terminals created by VS Code clients - #338612

Resolves: #338608

• Add a vscodeTerminalIdentity option to AgentHostTerminalManager.createTerminal that sets TERM_PROGRAM=vscode and TERM_PROGRAM_VERSION to the agent host's product version.
• Pass the requesting connection's AgentHostClientType from the createTerminal protocol handler to IAgentService.createTerminal.
• Enable the identity only for known VS Code clients (Editor Window or Agents Window), as identified by clientInfo at initialize.
• Keep the inherited environment for unknown clients, and for agent tool shells and ! commands, which call the terminal manager directly.
• Leave attach, reconnect, and ownership transfer unchanged, since they don't spawn a process.
• Keep the protocol, generated AHP files, and workbench code unchanged; the host decides from existing connection state.

Validation

• After a one-shot npm run transpile-client, agentHostTerminalManager.test.ts and protocolServerHandler.test.ts pass (192 tests), both with TERM_PROGRAM/TERM_PROGRAM_VERSION unset and with conflicting inherited values.
• Targeted ESLint on the changed files passed with no warnings.
• Full typechecking is left to CI.

Manual test

1. Start the updated build, open the Agents Window, and create a new Agent Host terminal.
2. Run printf 'TERM_PROGRAM=%s\nTERM_PROGRAM_VERSION=%s\n' "$TERM_PROGRAM" "$TERM_PROGRAM_VERSION".
3. Verify the output is vscode and the build's version.
4. Ask the agent to run echo "$TERM_PROGRAM" and verify the tool shell keeps the inherited value.

Inspirations from:

• Terminal identity variable names and values come from addTerminalEnvironmentKeys. Only the identity is shared, not the full workbench environment or extension-collection pipeline.
• The vscodeTerminalIdentity option shape and placement follow the existing per-request preventShellHistory and nonInteractive environment options in AgentHostTerminalManager.createTerminal.
• Passing per-connection client identity from the protocol handler to the agent service follows how dispatchAction passes client.telemetryContext to IAgentService.dispatchAction. Only its clientType is passed here, since terminal creation needs nothing else.
• Gating host behavior on the client type follows the Editor Window check for session working-directory actions, with clients identified by getAgentHostClientType.
• Using the host's product version follows RemoteTerminalChannel, which passes the server's productService.version when creating remote terminals.
• The protocol handler test's client-type assertion is copied semantically from session working-directory actions reach the agent service.