[pigeon] make swift enum CaseIterable

[HannesGitH]

instead of generating

enum FooEnum: Int {
  case foo = 0
  ...

we will now generate

enum FooEnum: Int, CaseIterable {
  case foo = 0
  ...

allowing consumers to use FooEnum.allCases to iterate over all possible enum values in swift platform code

fixes flutter/flutter#186447

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[gemini-code-assist]

Code Review

This pull request updates Pigeon to version 26.3.5 and adds CaseIterable conformance to generated Swift enums. Feedback indicates that the change lacks necessary tests or golden file updates to verify the generated code, as required by the style guide.

[gemini-code-assist]

The addition of CaseIterable to generated Swift enums is a useful improvement. However, I noticed that no new tests or updates to existing golden files were included in this pull request to verify this change. Please ensure that the generated output is covered by tests to prevent regressions and verify the correct syntax is produced.

[HannesGitH]

it is now included in test