🏗️ Architect: Refactor RequestExecutor for Modularity

[fderuiter]

Refactored the RequestExecutor class into a class hierarchy (BaseRequestExecutor, SyncRequestExecutor, AsyncRequestExecutor) to separate synchronous and asynchronous logic, adhering to the Single Responsibility Principle.

Changes:

• Created imednet/core/http/executor.py with BaseRequestExecutor, SyncRequestExecutor, and AsyncRequestExecutor.
• Updated imednet/core/http_client_base.py to use dependency injection for the executor class via EXECUTOR_CLS.
• Updated imednet/core/client.py to use SyncRequestExecutor.
• Updated imednet/core/async_client.py to use AsyncRequestExecutor.
• Updated tests to reflect these changes.

This change eliminates runtime is_async checks in the executor and allows for better type safety and extensibility.

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PR created automatically by Jules for task 6890428304471025114 started by @fderuiter

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