Fix ProtocolError exceptions bypassing download resume logic

[lets-build-an-ocean]

When a download is interrupted mid-stream by a sudden network drop, urllib3 raises a ProtocolError (wrapping an IncompleteRead). Because _process_response() only caught ReadTimeoutError, the ProtocolError propagated uncaught, skipping the is_incomplete() check in __call__() and never reaching _attempt_resumes_or_redownloads(). This means pip's built-in resume logic was never triggered.

Fixes #14079

What I changed:

• Added ProtocolError to the except clause in _process_response() so broken connections are handled the same as timeouts.
• Added ProtocolError to the retry loop's except clause in _attempt_resumes_or_redownloads() so resume attempts that also drop don't crash either.
• Updated the warning message to "Connection interrupted while downloading." since it now covers both timeouts and broken connections.

Testing:
Added a unit test that simulates a ProtocolError being raised mid-stream and verifies pip resumes and completes the download instead of crashing. Also verified manually using the reproduction script from the issue.

[sepehr-rs]

Hi @lets-build-an-ocean, thank you for your contribution!
On a quick look and local testing, this looks good and I can confirm the exception is handled correctly now. Just a heads up that it may be a little while before a maintainer can take a closer look at this, since we're all volunteers. Appreciate your patience!

[lets-build-an-ocean]

Hi @lets-build-an-ocean, thank you for your contribution! On a quick look and local testing, this looks good and I can confirm the exception is handled correctly now. Just a heads up that it may be a little while before a maintainer can take a closer look at this, since we're all volunteers. Appreciate your patience!

Thanks for the quick review and for confirming the fix works! No rush at all — I really appreciate the time you and the other volunteers put into this project.

[notatallshaw]

Hope you don't mind, but I've added some more extensive unit tests. Otherwise looks good.

[lets-build-an-ocean]

Hope you don't mind, but I've added some more extensive unit tests. Otherwise looks good.

Thank you for the additional tests and the approval. Much appreciated!