[flutter_tools] Fix flutter doctor path validation for symlinked SDKs - #191975

Description

Flutter's FlutterValidator resolves the executable found on PATH, but previously compared that resolved path with the unresolved Flutter SDK root.

On Windows, package managers such as Scoop expose the active SDK through a current Junction:

current\bin\flutter -> 3.47.2\bin\flutter

Both paths refer to the same SDK, but flutter doctor incorrectly reported Flutter and Dart as being outside the current SDK.

This change:

• Resolves the Flutter SDK root directory only for internal containment comparison.
• Preserves the logical SDK root for version information, warnings, and PATH suggestions.
• Catches FileSystemException when the root cannot be resolved and falls back to the existing lexical comparison.
• Preserves the existing case normalization, separator normalization, and separator-boundary checks.
• Does not modify Cache.flutterRoot, public APIs, or user-visible logical paths.

No screenshots are applicable because this change affects Flutter CLI validation output.

Issues Fixed

Fixes #191936

Testing

• flutter test --no-pub test/general.shard/flutter_validator_test.dart from packages/flutter_tools — 23 tests passed.
• flutter analyze from packages/flutter_tools — passed.
• dart format --output=none --set-exit-if-changed — passed.
• Verified with a Windows Scoop Junction using where.exe flutter, where.exe dart, and the patched doctor -v.
• Confirmed that both Flutter and Dart PATH warnings disappear while the other doctor checks remain unchanged.

The regression tests cover:

• Flutter and Dart binaries under a Windows-style directory link.
• Unrelated SDK paths, which must continue to produce warnings.
• SDK roots that cannot be resolved.
• Existing Windows case and separator behavior.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

If this change needs to override an active code freeze, provide a comment explaining why. The code freeze workflow can be overridden by code reviewers. See pinned issues for any active code freezes with guidance.

Note: The Flutter team is currently trialing the use of Gemini Code Assist for GitHub. Comments from the gemini-code-assist bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.