fix: add support for nullable arrays in Swift 5,6

[HarelM]

• Fixes [BUG] [SWIFT] Array items cannot be nullable #22355

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CC: @jgavris @ehyche @Edubits @jaz-ah @4brunu @dydus0x14

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Summary by cubic

Respect nullable array items in Swift 5 and 6 generators, and nullable map values in Swift 6, so generated types match OpenAPI nullability. Previously arrays/sets and maps used non-optional element/value types; now nullable items/values become optionals (e.g., [String?], Set<String?>, [String: Foo?]).

• Affected generators: Swift5ClientCodegen (arrays/sets) and Swift6ClientCodegen (arrays/sets and map values); non-nullable items are unchanged.
• Detects nullability via nullable: true, x-nullable, and $ref/aliased schemas, including nested collections.
• Breaking change: callers must handle nil elements and map values where schemas mark items as nullable.

^{Written for commit ecc7b3f. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 6 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

All reported issues were addressed across 6 files (changes from recent commits).

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>