[pigeon] finish kotlin and swift safe cast removal

[tarrinneal]

Removes safe casting during decode process for kotlin and swift to avoid hiding potential data type inconsistencies across channels.

fixes https://github.com/flutter/flutter/issues/116999

Pre-launch Checklist I read the Contributor Guide and followed the process outlined there for submitting PRs. I read the Tree Hygiene wiki page, which explains my responsibilities. I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.) 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 listed at least one issue that this PR fixes in the description above. I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes. I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style. 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. All existing and new tests are passing. If you need help, consider asking for advice on the #hackers-new channel on Discord.

[tarrinneal]

@stuartmorgan you were right that there was a missed null cast, I am still using the cast methods though, so the safeCast parameter is still used.

[tarrinneal]

Testing label to see if it lands a "stuck" pr, not to fix tree, but to "un-stick" a good to land pr on a green tree.