test: pin the 2.x me-403 contract; fix settings-override pollution

[tomwojcik]

PR #920 was merged into master before its fix commit was pushed to the branch, so master carried a weak test_me_returns_200_for_authenticated_user test instead of the required 403-contract test. This PR carries that fix commit forward:

1. Replaces the weak test with test_me_delete_returns_403_not_404_when_permission_denied, which pins that a denied /users/me/ request answers 403 (not 404) in 2.x. It uses DELETE instead of GET because DELETE's permission (user_delete) is resolved at request time via get_permissions(), so override_settings actually takes effect.
2. Fixes a one-line bug in djoser/conf.py's Settings._override_settings: dict-valued DJOSER overrides were merged into the shared default ObjDict object in place, so an override made in one test leaked into every later settings reload in the process. The new test's override_settings(DJOSER={"PERMISSIONS": ...}) call would otherwise pollute later tests (test_user_delete.py / test_user_me.py DELETE tests fail when the override test runs first, without this fix). The fix copies the existing dict before merging in the override.

Test plan

• uv run py.test testproject/testapp/tests/test_urls/test_method_parity.py::test_me_delete_returns_403_not_404_when_permission_denied testproject/testapp/tests/test_user_delete.py testproject/testapp/tests/test_user_me.py -q — 17 passed
• uv run py.test testproject -q — 221 passed

🤖 Generated with Claude Code

https://claude.ai/code/session_011uYLkj9YnPJ1sNE2Juek2m

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.33%. Comparing base (6059138) to head (26b83f2).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #921   +/-   ##
=======================================
  Coverage   99.33%   99.33%           
=======================================
  Files          27       27           
  Lines         908      908           
=======================================
  Hits          902      902           
  Misses          6        6           

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