Address flakes from using temp directory to run tests

[srujzs]

2172ba7 added support to run tests in a temporary directory.

This results in two flaky issues:

1. On Windows, build_daemon tests may fail to delete the
   temp directory because the process may have not been torn
   down yet, so it may still be accessing the file system.
   There was an initial retry after 1 second, but that appears
   to be not enough looking at a recent test run. See https://github.com/dart-lang/webdev/actions/runs/16157459808/job/45602784802?pr=2649
   for an example. It's printed as a warning.
2. If a test times out, its tearDown may not be called. frontend_server_common
   relies on modifying the current directory via dart:io. Without a tearDown,
   it never restores the directory the process was in. This
   leads to cascading failures in subsequent tests due to it no
   longer being in a path that contains 'webdev'. See
   https://github.com/dart-lang/webdev/actions/runs/15989286213/job/45099373212?pr=2641
   for an example. See tearDown is not called if a test times out test#897
   as well for tracking work to call tearDown on timeouts.

To address the above issues:

1. Add exponential backoffs to try again (up to a max number of attempts). Ideally, we'd be able to wait for some event instead, but it doesn't look like the daemon exposes the process nor does that necessarily mean Windows doesn't still have a lock on it.
2. Migrate frontend_server_common to not change the current working directory in the FileSystem and instead always use full URIs wherever needed.

[nshahan]

Looks like we still see an existing failure on the unit_test; windows; Dart dev; PKG: webdev; dart test -j 1. but I think that failure isn't expected to be fixed by this change. Can you confirm?

[srujzs]

Looks like we still see an existing failure on the unit_test; windows; Dart dev; PKG: webdev; dart test -j 1. but I think that failure isn't expected to be fixed by this change. Can you confirm?

Indeed, that's failing in the CI already and will require a separate PR: https://github.com/dart-lang/webdev/actions/runs/16243343947/job/45862639306

[bkonyi]

LGTM!

Updating things so we're not changing the CWD is a big improvement. I'm fairly certain this is something that we've been burned by in Flutter Tools tests since there's a warning if you try to change the CWD in a test.

Windows is a PITA when it comes to cleaning up resources after a test. As long as we don't crash or cause the test to fail, it's fine to occasionally leak these resources since they'll only persist as long as the container is alive anyway.

[nshahan]

Thanks for the stability improvements!

[srujzs]

Windows is a PITA when it comes to cleaning up resources after a test.

Indeed, and even with this PR, we're not guaranteed to free up the temp directory (as seen in the logs). But hopefully this reduces the number of instances where this occurs.

Thanks for the reviews!