[stable] [flutter_tools] Safely handle broken symlinks and existing files during plugin symlink creation - #194578

…ng plugin symlink creation (flutter#191496)

## Description

Fixes a crash during plugin symlink creation on Windows/Linux desktop
builds (`FileSystemException: Creation failed, OS Error: A file cannot
be created if it already exists., errno = 183` / `errno = 17`).

### Cause
Previously, `_createPlatformPluginSymlinks` checked `if
(link.existsSync()) continue;` before calling `link.createSync(path)`.
1. If a non-link entity (such as a regular directory or file from manual
extraction, git checkout, or previous builds) occupied the symlink path,
`Link.existsSync()` returned `false`. The tool then called
`link.createSync(path)`, crashing with `ERROR_ALREADY_EXISTS` (`errno =
183`) on Windows or `EEXIST` (`errno = 17`) on POSIX.
2. `Link.existsSync()` did not check if an existing symlink pointed to
the expected plugin target path. If a plugin's path changed (e.g. a
version upgrade in pub cache), existing symlinks pointing to outdated
paths were skipped.

### Fix
- Uses `typeSync(link.path, followLinks: false)` to inspect disk state
type-agnostically before symlink creation.
- Canonicalizes and compares `link.targetSync()` with `path`. If
matching, creation is skipped.
- Deletes stale or mismatched links via
`ErrorHandlingFileSystem.deleteIfExists(link)`, and removes conflicting
non-link entries before calling `link.createSync(path)`.

## Related Issues
* Fixes flutter#90028
* Related to flutter#131978
* Related to flutter#74227

## Tests
- `packages/flutter_tools/test/general.shard/plugins_test.dart`:
  - `createPluginSymlinks repairs broken symlinks without failing`
- `createPluginSymlinks replaces existing files with symlinks without
failing`