#24986 [Java][jersey2/jersey3] Fix serialization of array path parameters

Closes #24985

Array path parameters were serialized with Collection.toString(), producing "[a, b, c]" in the URL. Add ApiClient.collectionPathParameterToString, which escapes each item individually and joins them with the delimiter for the parameter's collection format (csv by default).

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Summary by cubic

Fixes Java jersey2/jersey3 clients serializing array path parameters as [a, b, c] (from Collection.toString()) instead of the collection format's delimiter. Adds collectionPathParameterToString to ApiClient, which escapes each item individually and joins them with the correct delimiter; multi falls back to csv since it isn't valid for path parameters.

• Preserves the delimiter before leading empty or null items (e.g. ["", "b"] becomes ",b").
• Updates generated samples and adds runtime tests for collectionPathParameterToString plus a codegen test using a new array path parameter spec.