test: cover syncContext context-key derivation + exit idempotency (CoeditingRegistry)

Implements comprehensive unit test coverage for CoeditingRegistry.syncContext,
which derives context keys (cowriting.isCoediting, cowriting.baselineMode) via
vscode.commands.executeCommand — previously untested. Added 8 new tests:

- isCoediting: true for active editor with coediting doc
- isCoediting: false for undefined editor or non-entered doc
- baselineMode: uses hook result when set; defaults to "" otherwise
- exit idempotency: double exit fires onDidChange only once

All assertions inspect the mocked executeCommand call arguments directly.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/coeditingRegistry.test.ts

@@ -14,6 +14,7 @@ vi.mock("vscode", () => ({
   commands: { executeCommand: vi.fn() },
   window: { activeTextEditor: undefined },
 }));
+import * as vscode from "vscode";
 import { CoeditingRegistry } from "../src/coeditingRegistry";
 
 function memento(): { store: Map<string, unknown> } & { get: any; update: any } {
@@ -60,4 +61,109 @@ describe("CoeditingRegistry", () => {
     reg.enter(uri);
     expect(events).toHaveLength(1);
   });
+
+  it("exit is idempotent (no duplicate events on double exit)", () => {
+    const reg = new CoeditingRegistry(memento() as any);
+    const events: unknown[] = [];
+    reg.onDidChange((e) => events.push(e));
+    const uri = { toString: () => "file:///a.md" } as any;
+    reg.enter(uri);
+    reg.exit(uri);
+    reg.exit(uri);
+    expect(events).toHaveLength(2); // enter, then exit (second exit fires nothing)
+  });
+
+  it("syncContext: active editor with coediting doc sets isCoediting=true", () => {
+    const reg = new CoeditingRegistry(memento() as any);
+    const uri = { toString: () => "file:///a.md" } as any;
+    reg.enter(uri);
+
+    const mockExecuteCommand = vscode.commands.executeCommand as any;
+    mockExecuteCommand.mockClear();
+
+    const editor = { document: { uri } } as any;
+    reg.syncContext(editor);
+
+    expect(mockExecuteCommand).toHaveBeenCalledWith("setContext", "cowriting.isCoediting", true);
+  });
+
+  it("syncContext: undefined editor sets isCoediting=false", () => {
+    const reg = new CoeditingRegistry(memento() as any);
+    const mockExecuteCommand = vscode.commands.executeCommand as any;
+    mockExecuteCommand.mockClear();
+
+    reg.syncContext(undefined);
+
+    expect(mockExecuteCommand).toHaveBeenCalledWith("setContext", "cowriting.isCoediting", false);
+  });
+
+  it("syncContext: non-coediting doc sets isCoediting=false", () => {
+    const reg = new CoeditingRegistry(memento() as any);
+    const mockExecuteCommand = vscode.commands.executeCommand as any;
+    mockExecuteCommand.mockClear();
+
+    const uri = { toString: () => "file:///nonexistent.md" } as any;
+    const editor = { document: { uri } } as any;
+    reg.syncContext(editor);
+
+    expect(mockExecuteCommand).toHaveBeenCalledWith("setContext", "cowriting.isCoediting", false);
+  });
+
+  it("syncContext: baselineMode defaults to empty string when not coediting", () => {
+    const reg = new CoeditingRegistry(memento() as any);
+    const mockExecuteCommand = vscode.commands.executeCommand as any;
+    mockExecuteCommand.mockClear();
+
+    const uri = { toString: () => "file:///a.md" } as any;
+    const editor = { document: { uri } } as any;
+    reg.syncContext(editor);
+
+    expect(mockExecuteCommand).toHaveBeenCalledWith("setContext", "cowriting.baselineMode", "");
+  });
+
+  it("syncContext: baselineMode uses hook result when coediting and hook is set", () => {
+    const reg = new CoeditingRegistry(memento() as any);
+    const uri = { toString: () => "file:///a.md" } as any;
+    reg.enter(uri);
+
+    reg.baselineModeOf = vi.fn((): "head" | "snapshot" | undefined => "snapshot");
+
+    const mockExecuteCommand = vscode.commands.executeCommand as any;
+    mockExecuteCommand.mockClear();
+
+    const editor = { document: { uri } } as any;
+    reg.syncContext(editor);
+
+    expect(mockExecuteCommand).toHaveBeenCalledWith("setContext", "cowriting.baselineMode", "snapshot");
+  });
+
+  it("syncContext: baselineMode defaults to empty string when hook returns undefined", () => {
+    const reg = new CoeditingRegistry(memento() as any);
+    const uri = { toString: () => "file:///a.md" } as any;
+    reg.enter(uri);
+
+    reg.baselineModeOf = vi.fn((): "head" | "snapshot" | undefined => undefined);
+
+    const mockExecuteCommand = vscode.commands.executeCommand as any;
+    mockExecuteCommand.mockClear();
+
+    const editor = { document: { uri } } as any;
+    reg.syncContext(editor);
+
+    expect(mockExecuteCommand).toHaveBeenCalledWith("setContext", "cowriting.baselineMode", "");
+  });
+
+  it("syncContext: calls executeCommand twice (isCoediting and baselineMode)", () => {
+    const reg = new CoeditingRegistry(memento() as any);
+    const uri = { toString: () => "file:///a.md" } as any;
+    reg.enter(uri);
+
+    const mockExecuteCommand = vscode.commands.executeCommand as any;
+    mockExecuteCommand.mockClear();
+
+    const editor = { document: { uri } } as any;
+    reg.syncContext(editor);
+
+    expect(mockExecuteCommand).toHaveBeenCalledTimes(2);
+  });
 });