fix(mcp): report missing ffmpeg distinctly from missing browser

[yury-s]

Summary

• The MCP launch error handler rewrote every Executable doesn't exist error into Browser "<channel>" is not installed, but the registry uses the same prefix for ffmpeg. With recordVideo enabled and ffmpeg missing, users were told to install a browser that was already there.
• Disambiguate by the executable path so ffmpeg surfaces its own message pointing at install-browser ffmpeg.

Fixes #40862

[github-actions]

Test results for "MCP"

9 failed
❌ [msedge] › mcp/annotate.spec.ts:230 › should capture annotations via show --annotate @mcp-windows-latest-msedge
❌ [msedge] › mcp/annotate.spec.ts:251 › should start dashboard and annotate when no dashboard is running @mcp-windows-latest-msedge
❌ [msedge] › mcp/annotate.spec.ts:273 › should enter annotate mode on fresh dashboard.tsx mount with -s --annotate @mcp-windows-latest-msedge
❌ [msedge] › mcp/annotate.spec.ts:427 › should switch screencast to -s session on show --annotate @mcp-windows-latest-msedge
❌ [msedge] › mcp/annotate.spec.ts:476 › should disengage annotate mode when --annotate client disconnects @mcp-windows-latest-msedge
❌ [msedge] › mcp/cli-devtools.spec.ts:217 › video-start-stop @mcp-windows-latest-msedge
❌ [msedge] › mcp/cli-session.spec.ts:56 › close-all @mcp-windows-latest-msedge
❌ [msedge] › mcp/cli-session.spec.ts:99 › session stops when browser exits @mcp-windows-latest-msedge
❌ [msedge] › mcp/cli-session.spec.ts:130 › workspace isolation - sessions in different workspaces are isolated @mcp-windows-latest-msedge

7094 passed, 1113 skipped

---

Merge workflow run.