Generate `@Deprecated` annotation for deprecated operations and schemas

[halotukozak]

Description

OpenAPI 3.0 allows marking operations, parameters, and schemas with deprecated: true. Currently this flag is ignored — generated code has no indication that something is deprecated. The generator should add Kotlin's @Deprecated annotation to affected classes and functions.

Acceptance criteria

• SpecParser extracts the deprecated flag from operations and schemas
• ClientGenerator adds @Deprecated annotation to generated suspend functions for deprecated operations
• ModelGenerator adds @Deprecated annotation to generated data classes for deprecated schemas
• Deprecated parameters get @Deprecated on the corresponding function parameter
• Tests cover parsing and generation of deprecated operations, schemas, and parameters