Fix file transfers ignoring connection 'max_retries'

[BigRoy]

Bug

File uploads and downloads ignore the connection's max_retries and can end up making zero attempts:

• con.set_max_retries(1) (or ServerAPI(..., max_retries=1)) has no effect on transfers; they always use the env variable / class default (3).
• With AYON_SERVER_RETRIES=0:
  • uploads crash: AttributeError: 'NoneType' object has no attribute 'raise_for_status'
  • downloads silently do nothing — no request, no error, empty/missing file.

Cause

_upload_file and _download_file_to_stream used self.get_default_max_retries() instead of self.max_retries, and looped for attempt in range(retries) without ensuring at least one attempt. REST requests (_do_rest_request) already clamp to at least 1.

Fix

Use max(self.max_retries, 1) in both transfer functions.

Reproduce

AYON_SERVER_RETRIES=0 python -c "
import ayon_api
con = ayon_api.get_server_api_connection()
print(con.create_thumbnail('<project>', '/path/to/image.png'))
"
# develop: AttributeError: 'NoneType' object has no attribute 'raise_for_status'
# this PR: <thumbnail id>

Download with zero retries:

import io, ayon_api
con = ayon_api.get_server_api_connection()  # with AYON_SERVER_RETRIES=0
stream = io.BytesIO()
con.download_file_to_stream(f"api/projects/<project>/thumbnails/<thumbnail id>", stream)
print(len(stream.getvalue()))  # develop: 0

Testing notes

• tests/test_transfer_max_retries.py: zero retries still uploads/downloads; max_retries=1 makes exactly one upload attempt.

Stacked PRs touching the same download/upload loop build on top of this branch.

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