Support array encoding for string-based enum types

[JialinHuang803]

Support array encoding for string-based enum types.

[Copilot]

Pull request overview

This pull request adds support for array encoding (CSV, pipe-delimited, space-delimited, newline-delimited) for string-based enum types in the TypeSpec TypeScript emitter. Previously, only plain string arrays supported collection format encoding.

Changes:

• Added isStringEncodableArrayValueType() helper function to validate array value types that can use collection format encoding
• Updated validation logic to accept both string types and string-based enum types for array encoding
• Enhanced deserialization logic to properly cast enum arrays with fixed members to the correct type
• Added comprehensive test coverage for various array encoding formats with enum and union types

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
packages/typespec-ts/src/modular/helpers/operationHelpers.ts	Implements support for string-based enum array encoding by adding type checking helper and updating deserialization logic
packages/typespec-ts/test/modularUnit/scenarios/models/serialization/modelPropertyArrayEncoding.md	Adds test cases for enum and union array encoding scenarios