[file_selector_android] Surface file-copy failures to Dart instead of crashing

[tonytonycoder11]

file_selector_android crashes when a selected file can't be copied to a readable location.

FileSelectorApiImpl.toFileResponse catches IOException/SecurityException from FileUtils.getPathFromCopyOfFileFromUri and sets the path to null, but then still builds a FileResponse with that null path. Since path is non-null in the Pigeon-generated FileResponse, this throws — a NullPointerException since the move to the Kotlin Pigeon generator (the issue reported it as an IllegalStateException on the older Java generator) — so the app crashes instead of surfacing an error.

When no path can be produced, toFileResponse now returns null. The callers already handle that by completing the result with an error (the same completeWithError("Failed to read file: …") path they use elsewhere when a file can't be read), so the failure now reaches Dart as a PlatformException instead of crashing the app.

I kept this scoped to the crash. The issue also suggests moving these branches onto the typed FileSelectorNativeException mechanism (like #8184); that would need a Pigeon regeneration and a Dart-facing API change, so I left it out here — happy to do it as a follow-up if that's the preferred direction.

The existing test that documented the crash (marked "Remove when fixed") now asserts the failure is surfaced, plus a matching test for the single-file openFile path.

Fixes flutter/flutter#159568

Pre-Review 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 page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under.
• All existing and new tests are passing.

[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 a crash in the Android file selector implementation when a selected file cannot be copied to a readable location (such as during a SecurityException). It updates FileSelectorApiImpl to return null when uriPath is null, allowing the failure to be surfaced to Dart, and updates the corresponding tests to verify this behavior. The review feedback suggests using primitive boolean[] arrays instead of boxed Boolean[] arrays in the tests to prevent potential NullPointerExceptions during unboxing in assertions.