feat(chat): add handoff discovery and execution actions

[eleanorjboyd]

Adds workbench.action.chat.getHandoffs and workbench.action.chat.executeHandoff along with the necessary tests.

workbench.action.chat.getHandoffs returns an object array (ICustomAgentInfo[]) which include the the handoffs available across custom agents (and built-in modes).

workbench.action.chat.executeHandoff executes actually doing the handoff in the current or specified session to the custom agent/mode defined.

[Copilot]

Pull request overview

Adds programmatic discovery and execution of chat handoffs, enabling external automation (commands) to enumerate handoffs across modes and trigger a selected handoff in a target chat session.

Changes:

• Added workbench.action.chat.getHandoffs to return structured handoff metadata across built-in and custom modes.
• Added workbench.action.chat.executeHandoff to resolve and execute a handoff (optionally scoped to a session and/or source agent).
• Refactored ChatWidget to expose an executeHandoff(...) method and added unit tests for ID generation, discovery, and execution.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/common/chatModes.ts	Introduces getHandoffId, handoff/mode info types, and buildCustomAgentHandoffsInfo for the discovery command.
src/vs/workbench/contrib/chat/browser/actions/chatExecuteActions.ts	Registers new commands for handoff discovery and execution.
src/vs/workbench/contrib/chat/browser/widget/chatWidget.ts	Exposes executeHandoff(...) for reuse by UI and commands.
src/vs/workbench/contrib/chat/browser/chat.ts	Extends IChatWidget interface with executeHandoff.
src/vs/workbench/contrib/chat/test/common/chatHandoffs.test.ts	Adds tests for getHandoffId and buildCustomAgentHandoffsInfo.
src/vs/workbench/contrib/chat/test/browser/actions/chatExecuteActions.test.ts	Adds browser tests covering both new commands and widget/session resolution behavior.

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[Copilot]

Pull request overview

Adds command-level APIs and supporting plumbing/tests for discovering and executing “handoff” actions across chat modes/agents, improving programmability of handoffs and strengthening prompt validation around handoff definitions.

Changes:

• Introduces workbench.action.chat.getHandoffs and workbench.action.chat.executeHandoff commands, plus shared handoff metadata helpers (getHandoffId, buildCustomAgentHandoffsInfo).
• Updates prompt parsing/validation to skip whitespace-only handoff labels and report duplicate handoff labels.
• Extends IChatWidget with an executeHandoff(...) method and adds/updates unit tests covering the new behaviors.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/common/chatModes.ts	Adds handoff ID generation and builds a structured handoff discovery payload (ICustomAgentInfo[]).
src/vs/workbench/contrib/chat/browser/actions/chatExecuteActions.ts	Adds the two new commands and wires them to mode/widget services for discovery + execution.
src/vs/workbench/contrib/chat/browser/chat.ts	Extends IChatWidget to expose executeHandoff(...) for command invocation.
src/vs/workbench/contrib/chat/browser/widget/chatWidget.ts	Implements executeHandoff(...) and routes handoff selection through it (with updated error logging).
src/vs/workbench/contrib/chat/common/promptSyntax/promptFileParser.ts	Skips parsing handoffs whose label is whitespace-only.
src/vs/workbench/contrib/chat/common/promptSyntax/languageProviders/promptValidator.ts	Adds duplicate-handoff-label validation.
src/vs/workbench/contrib/chat/test/common/promptSyntax/service/promptFileParser.test.ts	Adds coverage for skipping whitespace-only handoff labels.
src/vs/workbench/contrib/chat/test/common/chatHandoffs.test.ts	New unit tests for getHandoffId and buildCustomAgentHandoffsInfo.
src/vs/workbench/contrib/chat/test/browser/promptSyntax/languageProviders/promptValidator.test.ts	Adds coverage for duplicate handoff label diagnostics.
src/vs/workbench/contrib/chat/test/browser/actions/chatExecuteActions.test.ts	New unit tests for the new discovery/execution commands.

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/browser/widget/chatWidget.ts:1279

• acceptInput().catch(e => this.logService.error(...${e})) interpolates the error into the string, which typically loses stack/structured details. Prefer passing the error object as a separate argument to logService.error so the stack is preserved (and consider keeping the message free of ${e}).

			this.acceptInput().catch(e => this.logService.error(`[Handoff] Failed to submit delegated handoff to '@${agentId}': ${e}`));

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