Validate that '@param' comments on enumerators match a real field of that enumerator.

[InsertCreativityHere]

Currently slicec doesn't validate that @param tags on enumerators are for real fields. For example, this produces no lints:

enum Foo {
    /// @param fakename: this field doesn't exist.
    A(hello: int32)
}

This PR directly fixes #820, and is also the last piece of validation we were missing to consider #684 fully implemented.

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What's Changed:

• slicec: slicec now validates that @param tags on enumerators reference an existent field of the enumerator, like it already did for operation parameters.

[Copilot]

🟢 Approval recommended

The implementation is correct and well-tested; only a minor test-name typo remains.

Pull request overview

Adds validation ensuring enumerator @param tags reference actual fields.

Changes:

• Validates enumerator parameter documentation.
• Adds valid and invalid comment tests.

File summaries

File	Description
slicec/src/validators/enums.rs	Implements enumerator tag validation.
slicec/tests/comment_tests.rs	Covers accepted and rejected tags.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Balanced

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