Add configurable Swift attributes for generated APIProtocol

[farzadshbfn]

Motivation

Generated client and server code often needs Swift attributes on the APIProtocol type and its operation requirements—for example @MainActor for concurrency isolation or macro-driven attributes such as @Mockable. Today there is no supported way to express those attributes in generator configuration, so adopters must post-process generated Types.swift or maintain local forks.

Modifications

• Add AttributeConfiguration and AttributeDescription to model optional Swift attributes in config.
• Extend Config, _UserConfig, and CLI GenerateOptions with an attributes field.
• Decode YAML attributes: using protocol and methods keys (each a list of attribute names or quoted strings with arguments).
• Extend the structured Swift representation and text renderer to emit attribute declarations.
• Apply configured attributes when translating APIProtocol and its method requirements in Types.swift; OpenAPI-driven @available(*, deprecated, …) remains unchanged and is still applied from operation metadata.
• Document that consumers remain responsible for wiring any required dependencies in Package.swift, macro plugins, and additionalImports when attributes reference external types or macros.

Result

Adopters can configure attributes in openapi-generator-config.yaml (or equivalent) and regenerate Types.swift with the requested @ attributes on APIProtocol and each operation requirement, without manual edits to generated output.

Example:

attributes:
  protocol:
    - MainActor
  methods:
    - MainActor

Test Plan

• Added Test_AttributeConfiguration for YAML decoding of protocol / methods, including quoted attribute arguments.
• Extended Test_Config to cover the new configuration surface.
• Added testPaths_withConfiguredAttributes snippet reference test for generated APIProtocol output.
• Ran swift test --filter OpenAPIGeneratorCoreTests (116 tests, all passing).

Related issues

• Ability to attach macros to generated types and methods #383 — Request to attach macros (and similar attributes) to generated types and methods; this change adds configuration-driven attributes on APIProtocol and its operation requirements.
• Xcode 26 Beta 3 - Main actor-isolated conformance cannot satisfy conformance requirement for a 'Sendable' type parameter 'Self' #796 — Build failures under Xcode 26 default MainActor isolation; configuring @MainActor on generated APIProtocol and methods is a supported way to align generated code with project concurrency settings.