Restore TCP read backpressure after a cancelled receive

[Master-Norna]

NOTE Erasing or replacing the contents of this template will result in your pull
request being summarily closed without consideration!

Changes

Fixes #1423.

Pause transport reads in a finally block when leaving the receive wait.
Cancellation previously skipped this step, allowing incoming TCP data (including
TLS ciphertext) to accumulate without an active receiver. The fix preserves
cancellation and buffered data without closing the connection.

The regression test cancels receive() and checks that the transport has
paused reading, using the simpler test suggested in review.

Local validation of the simplified test (Windows, CPython 3.12):

• All 3 IPv4 asyncio configurations fail without the fix and pass with it.
• Related TCP/stream tests: 558 passed, 142 deselected after excluding IPv6
  and the two known environment-failing test groups.
• The 8 IPv6-related connection failures and 75 native-Windows mypy diagnostics
  are unchanged from the pre-simplification PR baseline. Other pre-commit hooks pass.
• Linux/macOS runtime tests, Python 3.10, uvloop, and the full repository suite
  were not rerun for this simplified test.

No documentation update is needed: this restores the intended backpressure behavior.

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) which would fail without your patch
• You've updated the documentation (in docs/), in case of behavior changes or new
  features
• You've added a changelog fragment (in changelog.d/).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

Updating the changelog

Add a news fragment file in the changelog.d/ directory, named
<issue or PR number>.<type>.rst, where <type> is one of:

• breaking: backwards incompatible changes
• added: new features
• changed: other user-visible changes
• fixed: bug fixes

If, say, your patch fixes issue #123, create changelog.d/123.fixed.rst
containing only a brief summary of the change, like this:

Fixed big bad boo-boo in task groups

The issue link and the credit for your pull request are added automatically on release.
If there's no issue linked, name the file after your pull request instead by adding the
fragment after you've created the PR.

[agronholm]

Thanks.