[url_launcher] Fix supportsCloseForLaunchMode to query close support

[NikhilKukreja26]

Fixes flutter/flutter#192758

supportsCloseForLaunchMode was calling UrlLauncherPlatform.supportsMode
(the launch-support check) instead of supportsCloseForMode, so it reported
whether a mode can be launched rather than whether an in-app web view opened
with that mode can be closed. This changes it to call supportsCloseForMode,
and adds a regression test (the shared mock is extended so the two platform
methods can return different values).

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.

[gemini-code-assist]

Code Review

This pull request updates supportsCloseForLaunchMode to correctly query supportsCloseForMode on the platform instead of supportsMode, alongside a version bump to 6.3.3 and a new regression test. Feedback recommends allowing setCloseForModeResponse to accept a nullable bool? and adding a tearDown block to reset the mock state, ensuring proper test isolation and preventing test pollution.

[stuartmorgan-g]

Thanks for the contribution!

[NikhilKukreja26]

Thanks for the review! Addressed all points — fresh mock via setUp, removed the fallback and nullable setter, and trimmed the CHANGELOG.