[dart] fix: handle multiline x-enum-descriptions

[Mattias-Sehlstedt]

Fixes so that dart can handle multiline x-enum-descriptions. This is done by changing the doc to /** ... */ and introducing a lambda to prefix each line with a *. Note that * is technically superfluous, and it is for example not used in the Java handling.

Reverted strange indent changes to enum.mustache from here, and instead made it a two-step indent as per every other definition in the template.

Fixes #24858

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Summary by cubic

Fixes the Dart generator emitting broken doc comments for multiline x-enum-descriptions by switching from /// to /** ... */ block comments and prefixing each line with * via new Mustache lambdas.

Details

• Adds PrefixLambda and the dartdoc/dartdoc_2 lambdas in AbstractDartCodegen.
• Uses block comments in the built_value and json_serializable enum templates.
• Adds multiline enum descriptions to the Dart petstore test spec and regenerates samples.
• Resolves [BUG] dart-dio generates invalid code for multi-line enum descriptions as of 7.25.0 #24858.

^{Written for commit 7073cc2. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 9 files

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[cubic-dev-ai]

All reported issues were addressed across 13 files (changes from recent commits).

<sub>Tip: Review your code locally with the cubic CLI to iterate faster.

Re-trigger cubic</sub>

[wing328]

thanks for the fix

cc @jaumard (2018/09) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)