[pigeon] Encode custom enum as long in Kotlin generator

[devgianlu]

This PR ensures that custom enums are encoded as the expected Long type in the Kotlin generator.

Without this change it is not possible to perform encoding <-> decoding in Kotlin, because enums are generated to have an Int raw value and encoded without first casting to Long.

Fixes: flutter/flutter#176081

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• 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 updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption 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.

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Footnotes

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[gemini-code-assist]

Code Review

This pull request addresses a ClassCastException in the Kotlin generator by ensuring custom enums are encoded as Long instead of Int. The core change correctly adds a .toLong() call during the serialization of enum raw values. The PR is well-structured, including updates to the version number in pubspec.yaml and generator_tools.dart, a clear entry in CHANGELOG.md, and a new test expectation to verify the fix and prevent regressions. The changes are correct and effectively resolve the reported issue.