[SPIKE] Revise MV3 testing plan

Planning the following work

- Tests running on mv2 vs mv3 builds and transition plan. (see https://app.zenhub.com/workspaces/metamask-extension-63529dce65cbb100265a3842/issues/gh/metamask/metamask-extension/18442)
- detail each phase
- Manual QA validation
- Unscripted QA (using a private link to beta from internal users)
- Hardware wallet manual tests
- Opening metamask and leave it running (see https://app.zenhub.com/workspaces/metamask-extension-63529dce65cbb100265a3842/issues/gh/metamask/metamask-extension/21348)
- Log user out timeout (functionality that changed from setInterval and setTimeout into chromeAlarms)
- Ensure snaps work well with mv3 (https://app.zenhub.com/workspaces/metamask-extension-63529dce65cbb100265a3842/issues/gh/metamask/metamask-extension/21071)
- Ensure all chromium based browsers support mv3 (https://app.zenhub.com/workspaces/metamask-extension-63529dce65cbb100265a3842/issues/gh/metamask/metamask-extension/21070)

Start by reviewing the linked issues 18442, 21348, 21071, and 21070 to understand the MV2/MV3 transition, long-running extension behavior, Snaps, and Chromium browser coverage. Done means documenting each testing phase, manual QA scenario, timeout validation, Snaps check, and browser-support check in a complete transition plan.

Written by the indexing model from the issue text.