[path_provider] Document null vs UnsupportedError return semantics

[theprantadutta]

Updates the dartdoc for getDownloadsDirectory in path_provider and the matching getDownloadsPath contract on PathProviderPlatform to explain both return-value cases:

• null — the platform supports the concept of a downloads directory but no such directory is currently available (concrete example: Linux without xdg-user-dir installed, or where xdg-user-dir fails when called).
• UnsupportedError — the current platform has no concept of a downloads directory at all.

Previously the dartdoc only mentioned the UnsupportedError case, which left users unsure when they might see a null result.

The wording matches the explanations already given by maintainers in the linked issue thread.

Fixes flutter/flutter#143238

Pre-Review Checklist

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• All existing and new tests are passing.

This is a documentation-only change to public dartdoc comments. No code behavior changes; no new tests are required per the auto-exempt rules for changes that only affect comments/documentation.

[gemini-code-assist]

Code Review

This pull request documents the difference between a null return and an UnsupportedError for getDownloadsDirectory and getDownloadsPath across the path_provider and path_provider_platform_interface packages. The review feedback points out a formatting issue in the CHANGELOG.md of path_provider, where inserting a new bullet point accidentally changes the nesting hierarchy of an existing sub-bullet point.

[gemini-code-assist]

Inserting this bullet point here accidentally changes the nesting of the subsequent bullet point. The sub-bullet * Applications built with older versions of Flutter... (on lines 8-9) was meant to be nested under * Updates README..., but now it is nested under this new bullet point.

To preserve the correct hierarchy, please move this new bullet point to the end of the ## NEXT section (after the nested bullet point) or place it before the * Updates README... bullet point.

[stuartmorgan-g]

Thanks for the contribution!

In the future, please do not delete the checklist that is in the PR template; it is there for a reason. This PR is missing required elements described in the checklist, which need to be addressed before it moves forward with review.

I am marking the PR as a Draft. Please re-add and complete the checklist, updating the PR as appropriate, and when that’s complete please feel free to mark the PR as ready for review.

Also, in the future please do not ping people in PR descriptions; it's extremely disruptive because it not only pings them on the PR (which is not constructive), it pings them again when the PR lands, and when it's mirrored into forks.

If you had to change anything in the [flutter/tests] repo, list the PRs here.

This is part of a different repository's template, which suggests that you used AI to create this entire PR description. Please don't do that. We have a template.

[theprantadutta]

Thanks for the careful review — all addressed:

• PR template: restored with the checklist completed (also removed the stray [flutter/tests] line that was from a different repo's template).
• @-mentions: removed from the description.
• CHANGELOG nesting: fixed in 8491953 so the Applications built with older versions… sub-bullet is correctly nested under the README bullet again. The new docs entry is moved to the end of the section.
• Version bumps: path_provider 2.1.5 → 2.1.6 and path_provider_platform_interface 2.1.2 → 2.1.3, with ## NEXT renamed to the new version per the continuous-release CHANGELOG conventions.

Marking ready for review.

[gemini-code-assist]

Code Review

This pull request updates the documentation for getDownloadsDirectory in path_provider and getDownloadsPath in path_provider_platform_interface to clarify the behavior and implementation contracts for returning null versus throwing an UnsupportedError. It also bumps the package versions to 2.1.6 and 2.1.3 respectively, and updates their changelogs. There are no review comments, and I have no feedback to provide.