Fix RangeError when stripping @docImport directives with non-ASCII characters and CRLF

[wyrindev]

Fixes #4180

Description

This PR fixes an unhandled RangeError during comment precaching when stripping @docImport directives in documentation comments containing non-ASCII characters (e.g. em-dash — or emojis) and CRLF line endings.

• Defensive bounds clamping: Clamped substring slice bounds (safeStart and safeEnd) to [0, content.length] in DocumentationComment._stripDocImports so that string offset drift never causes a fatal RangeError.
• Precache error diagnostics: Added a try-catch block around processComment() in DocumentationComment.precacheLocalDocs that logs the affected element location (location) and full identifier (fullyQualifiedName) with logWarning before rethrowing, avoiding opaque ParallelWaitError failures during batch doc precaching.
• Regression test: Added test_docImport_withNonAsciiAndCrlf in test/documentation_comment_test.dart (stress testing with 30 accumulated lines of comments containing em-dashes and CRLF line endings) to verify that doc imports are stripped cleanly without error.
• Changelog: Added an entry under ## 9.0.10-wip in CHANGELOG.md.

Verification

• Formatted via dart format.
• Analyzed via dart analyze (0 issues).
• Ran all 50 tests in test/documentation_comment_test.dart (50/50 passed).

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

Note: The Dart team is trialing Gemini Code Assist. Don't take its comments as final Dart team feedback. Use the suggestions if they're helpful; otherwise, wait for a human reviewer.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gemini-code-assist]

Code Review

This pull request fixes an unhandled RangeError that occurs during comment precaching when stripping @docImport directives containing non-ASCII characters or CRLF line endings. It resolves this by clamping the substring offsets to safe bounds within the content length. Additionally, it wraps the local documentation precaching in a try-catch block to log warnings with element location and exception details upon failure, and adds a test case to verify the fix. There are no review comments, so I have no feedback to provide.