[swift6] Add nonisolatedModels option for default MainActor isolation

[danieldickison]

This PR was generated by Claude Code but I've looked through the changes and it all looks reasonable to me.

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Fixes #24903

Adds a nonisolatedModels boolean option (default: false) to the swift6 generator.

Swift 6.2 lets a module opt into main-actor isolation by default (SE-0466, -default-isolation MainActor; SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor in Xcode 26). When generated sources are vendored into such a module, every model becomes @MainActor-isolated, so their Codable conformances can't be used from URLSession completions, background tasks, or any other nonisolated context ("conformance of 'Foo' to protocol 'Decodable' crosses into main actor-isolated code").

With nonisolatedModels: true, the generator marks the following nonisolated:

• model struct/final class/enum declarations (including the objcCompatible NSObject subclasses), and the Identifiable / UnknownCaseCheckable extensions emitted next to them
• the protocols and helper types models conform to or store: ParameterConvertible, CaseIterableDefaultsLast, UnknownCaseCheckable, NullEncodable, JSONValue, OpenAPIDateWithoutTime, the validation rule structs
• the ParameterConvertible conformances and the CodingKey / keyed-container extensions in Extensions.swift
• CodableHelper, OpenAPIMutex and OpenISO8601DateFormatter, which the above depend on

Output is byte-identical when the option is off (all 14 swift6 sample configs were regenerated; only validation, where the option is now enabled, changed). Enabling the option requires a Swift 6.1+ toolchain (nonisolated on types and extensions, SE-0449).

Scope note: the API layer (APIs.swift, URLSessionImplementations.swift, RequestTask) is unchanged and still assumes a nonisolated module. This option targets the model layer, per the issue.

Note on objcCompatible models: the option is isolation-neutral for them. Those classes are already declared @unchecked Sendable with mutable properties, and nonisolated only removes a main-actor guarantee that Objective-C callers never honored anyway. It does not make them more or less thread-safe than they are today; the safe usage pattern (decode, then treat as read-only when shared, or use readonlyProperties) is unchanged.

Testing

• Swift6* unit tests pass (50 run, 0 failures), including two new generation tests asserting the nonisolated output with the option on and its absence with the option off.
• Built the regenerated validation sample and the other swift6 samples with Swift 6.4: all 13 directories in the samples-swift6.yaml macOS matrix (alamofireLibrary, apiNonStaticMethod, asyncAwaitLibrary, combineDeferredLibrary, combineLibrary, default, objcCompatible, oneOf, promisekitLibrary, resultLibrary, rxswiftLibrary, urlsessionLibrary, validation) pass swift build.
• Compiled the validation, petstore-with-fake-endpoints, recursive-models and oneOf outputs with .defaultIsolation(MainActor.self) in a scratch package (API layer excluded): 0 diagnostics in model and support files with the option on. With the option off, the same builds fail with e.g. conformance of 'Banana' to 'Decodable' cannot be used in nonisolated context and conformance of 'EnumClass' to protocol 'CaseIterableDefaultsLast' crosses into main actor-isolated code.
• ./mvnw verify on modules/openapi-generator: BUILD SUCCESS (checkstyle, jacoco check, javadoc, forbiddenapis all pass); surefire 5134 run, 0 failures, 12 skipped. On my local JDK 25 there are 8 errors in KotlinTestUtilsTest / KotlinSpringServerCodegenTest (the embedded Kotlin compiler can't parse the JDK version string 25.0.3); those two classes pass on JDK 21 (288 run, 0 failures, 0 errors), and this change doesn't touch them.

PR checklist

• Read the contribution guidelines.
• Ran ./mvnw clean package, ./bin/generate-samples.sh bin/configs/swift6-*.yaml and ./bin/utils/export_docs_generators.sh; all changed files are committed.
• Swift technical committee: @jgavris @ehyche @Edubits @jaz-ah @4brunu @dydus0x14

🤖 Generated with Claude Code

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Summary by cubic

Adds a nonisolatedModels option to the swift6 generator so generated sources stay usable when vendored into modules built with Swift 6.2 default MainActor isolation (SE-0466). Fixes #24903.

• When on, marks model declarations and their Identifiable/UnknownCaseCheckable extensions, supporting protocols and helpers, and objcCompatible classes nonisolated; requires a Swift 6.1+ toolchain.
• Off by default with byte-identical output; the API layer is unchanged and still assumes a nonisolated module.
• Enabled in the swift6 validation sample, with generation tests asserting the option's effect on and off, covering enum, oneOf, and objcCompatible models.

^{Written for commit 37202c0. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 26 files

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

Re-trigger cubic</sub>

[4brunu]

Can you please take a look at the comments? Thanks

[cubic-dev-ai]

All reported issues were addressed across 2 files (changes from recent commits).

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

Re-trigger cubic</sub>