[pigeon] Make Kotlin FlutterError a runtime error

[stuartmorgan-g]

Change the Kotlin generator's FlutterError to inherit from RuntimeException, rather than just Runnable, so that it behaves as an unchecked error when used from Java (as is the case for the Java version).

Fixes flutter/flutter#184200

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 under¹.
• 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 under¹.
• All existing and new tests are passing.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request updates the Pigeon generator to make generated error classes inherit from RuntimeException instead of Throwable, improving Java interoperability. The change includes updates to the Kotlin generator, platform test files, and the package version. A test case was added to verify the inheritance, and a suggestion was provided to make the test assertion more robust by specifically targeting the FlutterError class definition.

[tarrinneal]

This isn't breaking at all?

[stuartmorgan-g]

This isn't breaking at all?

RuntimeException inherits from Throwable, so anything assuming it's a Throwable will still be fine. I checked that throws FlutterError as an annotation is fine even for a RuntimeException (it's just no longer required). I couldn't think of any plausible cases that would be broken.