Minor improvements in api_client.py

[wing328]

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Summary by cubic

Use http.client header getters throughout ApiClient and sanitize downloaded filenames to prevent directory traversal. Regenerated Python samples.

• Bug Fixes
  • Read Content-Type/Disposition via response.getheader; return headers via response.getheaders in ApiResponse.
  • Sanitize filename from Content-Disposition with os.path.basename to prevent directory traversal.
  • Regenerated all Python samples (sync, aiohttp, httpx, lazy imports, echo_api).

^{Written for commit a022b90. Summary will update on new commits.}