aiohttp 3.13.4 rejects duplicate Server headers from Azure Blob Storage

[tbak-openai]

Describe the bug

Hi aiohttp maintainers,

We hit a production regression after upgrading to aiohttp 3.13.4, which includes #12240. Requests to Azure Blob Storage that worked in 3.13.3 now fail with:

ClientResponseError: 400, message="Duplicate 'Server' header found."

I understand this change is intentional and RFC-motivated, but it is a breaking change in a patch release for clients talking to real-world servers that emit duplicate singleton response headers.

Impact:
This breaks Azure Blob Storage access for environments that pick up aiohttp 3.13.4 transitively.

To Reproduce

Repro:

1. Send an authenticated GET to an Azure Blob container listing endpoint:
   https://<account>.blob.core.windows.net/<container>?restype=container&comp=list&maxresults=1
2. The response is 200 OK, but includes two Server headers:
   Server: Microsoft-HTTPAPI/2.0
Server: Windows-Azure-Blob/1.0 Microsoft-HTTPAPI/2.0
3. aiohttp 3.13.3 accepts this response.
4. aiohttp 3.13.4 rejects it with Duplicate 'Server' header found.

Expected behavior

Would you consider one of the following for 3.13.x?

• restoring tolerant handling for duplicate singleton response headers
• adding an opt-out / compatibility mode for strict response header validation
• documenting this as a backward-incompatible change with a recommended client
  workaround

Logs/tracebacks

curl -v https://<account_name>.blob.core.windows.net/<container>\?restype\=container\&comp\=list\&maxresults\=1

Response:
HTTP/1.1 ...
...
< Server: Microsoft-HTTPAPI/2.0
< Server: Blob Service Version 1.0 Microsoft-HTTPAPI/2.0
...

Python Version

$ python --version
Python 3.12.9

aiohttp Version

$ python -m pip show aiohttp
Name: aiohttp
Version: 3.13.4
Summary: Async http client/server framework (asyncio)
Home-page: https://github.com/aio-libs/aiohttp
Author:
Author-email:
License: Apache-2.0 AND MIT
Location: .../lib/python3.12/site-packages
Requires: aiohappyeyeballs, aiosignal, attrs, frozenlist, multidict, propcache, yarl
Required-by: #N/A

multidict Version

$ python -m pip show multidict

propcache Version

$ python -m pip show propcache

yarl Version

$ python -m pip show yarl

OS

macOS

Related component

Client

Additional context

No response

Code of Conduct

• I agree to follow the aio-libs Code of Conduct