fix(mcp): default stdio sessions to isolated

[BaseInfinity]

Summary

Default stdio MCP sessions to isolated browser contexts unless the user explicitly opts into persistence or an attached browser mode.

This avoids default-profile SingletonLock collisions for short-lived stdio MCP clients after a previous client disconnects without an explicit browser_close. HTTP/SSE transports keep the existing persistent default.

Fixes #40419.

Changes

• Resolve stdio MCP defaults to browser.isolated = true when no persistence or attachment mode is configured.
• Keep HTTP/SSE, --user-data-dir, remote endpoint, CDP endpoint, extension, and explicit --isolated behavior unchanged.
• Update MCP docs to describe the stdio-vs-HTTP/SSE profile defaults.
• Update MCP tests whose tab/root expectations depend on the default profile mode.
• Keep tracing start/stop on the same isolated browser context when tracing starts before navigation.

Verification

• RED: 5dafdf939 test(mcp): cover stdio default isolated behavior
• GREEN: 52afd9c76 fix(mcp): default stdio sessions to isolated
• npm run build
• npm run ctest-mcp -- tests/mcp/config-resolve.spec.ts tests/mcp/launch.spec.ts tests/mcp/profile-lock.spec.ts tests/mcp/roots.spec.ts tests/mcp/tabs.spec.ts tests/mcp/tracing.spec.ts
  • 87 passed, 1 skipped
• env -u NO_COLOR npm run ctest-mcp -- --workers=3
  • 589 passed, 3 skipped
• npm run flint

Note: I unset NO_COLOR locally only to avoid a Node 25 color-environment warning affecting exact-output MCP tests.

[BaseInfinity]

@microsoft-github-policy-service agree

[pavelfeldman]

That's unfortunately very breaking.