Vitest browser mode breaks ensureFileWatched (commonly breaking pnp virtual)

Describe the bug

Files in vitest that would normally be watched by vitest here:

https://github.com/vitejs/vite/blob/30194fa1e41dda6470aa20f2bb34655c4bfd9cd1/packages/vite/src/node/server/transformRequest.ts#L286-L292

are not watched when running in browser mode, as environment.pluginContainer.watcher is a no-op watcher. This will apply in fact to any attempt to watch a file via the PluginContainer.

Reproduction

https://github.com/mrginglymus/vitest-pnp

System Info

System:
    OS: Windows 10 10.0.19045
    CPU: (24) x64 Intel(R) Core(TM) i9-10920X CPU @ 3.50GHz
    Memory: 20.97 GB / 63.80 GB
  Binaries:
    Node: 22.13.0 - C:\Program Files\nodejs\node.EXE
    Yarn: 4.5.2 - C:\Program Files\nodejs\yarn.CMD
    npm: 10.9.2 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: Chromium (131.0.2903.146)
    Internet Explorer: 11.0.19041.4355

Used Package Manager

yarn

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

Start with the browser-mode environment.pluginContainer.watcher behavior and compare it with Vite's transformRequest.ts lines 286-292. Use the vitest-pnp reproduction to verify the failure, and consider the work complete when files passed through the PluginContainer are watched in browser mode and the reproduction works.