fix(web): apply retry handlers to files_upload_v2 file POST - #1941

Summary

files_upload_v2 has three steps. Steps 1 and 3 (files.getUploadURLExternal / files.completeUploadExternal) go through api_call() and therefore honor the client's retry handlers. Step 2 — the raw POST to the files.slack.com upload URL — called _upload_file_via_v2_url() directly, so transient HTTP errors (the reporter sees 2–6 HTTP 504s per day) escaped to the caller.

This change runs that POST through the same retry-handler loop used by api_call(). Existing handlers (including custom ones that retry 504) now apply. When no handler matches, the original HTTPError is still raised.

Fixes #1853

Testing

• pytest tests/slack_sdk/web/test_web_client_file_upload_retry.py
  • 504 then 200 with a 5xx retry handler succeeds
  • 504 with no handlers still raises HTTPError
• pytest tests/slack_sdk/web/test_web_client_http_retry.py tests/slack_sdk/web/test_web_client_http_retry_server_error.py tests/slack_sdk/http_retry/test_builtins.py (existing retry tests, still pass)

Category

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs (Documents)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run the new unit tests and the existing WebClient retry tests after making the changes.

Note: I did not run the full ./scripts/run_validation.sh matrix locally. Async files_upload_v2 already used retry handlers; this PR only changes the sync path, as confirmed in #1853.