Fix mutable morsels - #13637

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.02%. Comparing base (fbc5851) to head (0245bd0).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

@@            Coverage Diff             @@
##           master   #13637      +/-   ##
==========================================
- Coverage   99.02%   99.02%   -0.01%     
==========================================
  Files         135      135              
  Lines       50818    50845      +27     
  Branches     2671     2674       +3     
==========================================
+ Hits        50325    50351      +26     
  Misses        370      370              
- Partials      123      124       +1     

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Confidence Score: 5/5

Safe to merge based on direct ownership-isolation checks, real HTTP cookie delivery, and focused regression coverage.

The focused checks reproduced the previous mutable-object behavior on the base revision and confirmed the corrected behavior on this revision, including raw-header cookie handling and abstract/concrete cookie-jar compatibility.

Files Needing Attention: No files need further attention; aiohttp/cookiejar.py and tests/test_cookiejar.py were exercised directly.

_{Reviews (1): Last reviewed commit: "Rename 13634.bugfix.rst to 13637.bugfix...." | Re-trigger Greptile}

Merging this PR will not alter performance

✅ 97 untouched benchmarks
⏩ 83 skipped benchmarks¹

_{Comparing fix-mutable-morsel (0245bd0) with master (fbc5851)}

Backport to 3.15: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 1857e6d on top of patchback/backports/3.15/1857e6df6e9efce10dcf08a4ee26a366add2e443/pr-13637

Backporting merged PR #13637 into master

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   git remote add upstream https://github.com/aio-libs/aiohttp.git
   

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   git fetch upstream
   git checkout -b patchback/backports/3.15/1857e6df6e9efce10dcf08a4ee26a366add2e443/pr-13637 upstream/3.15
   

4. Now, cherry-pick PR Fix mutable morsels #13637 contents into that branch:

   git cherry-pick -x 1857e6df6e9efce10dcf08a4ee26a366add2e443
   

   If it'll yell at you with something like fatal: Commit 1857e6df6e9efce10dcf08a4ee26a366add2e443 is a merge but no -m option was given., add -m 1 as follows instead:

   git cherry-pick -m1 -x 1857e6df6e9efce10dcf08a4ee26a366add2e443
   

5. At this point, you'll encounter some merge conflicts. You must
   resolve them in order to preserve the patch from PR Fix mutable morsels #13637 as close to the
   original as possible.
6. Once conflicts are resolved and git added, run:

   git cherry-pick --continue
   

7. Push this branch to your fork on GitHub:

   git push origin patchback/backports/3.15/1857e6df6e9efce10dcf08a4ee26a366add2e443/pr-13637
   

8. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

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