[stable] [tool] Fix Windows hot reload mtime truncation issue - #191599

On Windows, `dart:io` reports `FileStat.modified` truncated to second
precision (via `GetFileAttributesExW`). When
`ProjectFileInvalidator.findInvalidated` compares file modification
times with `lastCompiled` (which is recorded with millisecond
precision), files edited in the same second as the previous compilation
are ignored because `updatedAt.isAfter(lastCompiled)` returns `false`.

This PR fixes the issue by truncating `lastCompiled` to second precision
on Windows and using `!updatedAt.isBefore(lastCompiledTruncated)` to
ensure files modified within the same second as the compilation start
time are marked as invalidated.

Fixes flutter#191198

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
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responsibilities.
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we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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---------

Co-authored-by: Loïc Sharma <[email protected]>