[tool] Fix Windows hot reload mtime truncation issue (#191271)

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 https://github.com/flutter/flutter/issues/191198

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
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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https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
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[Features we expect every widget to implement]:
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[CLA]: https://cla.developers.google.com/
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

---------

Co-authored-by: Loïc Sharma <737941+loic-sharma@users.noreply.github.com>