Preserve wrapper identity during in-place dictionary union

[Gonghan-Princess]

Describe your changes

After b = benedict(source); b |= other, b currently becomes the underlying plain dict. Keypath access and the original wrapper identity are lost even though the source dictionary is updated correctly. For example, a wrapper configured with keypath_separator="/" can no longer resolve b["item/name"] after the union.

Keep the existing in-place update of the wrapped dictionary and return the wrapper itself. Add regressions for dictionary and key/value iterable inputs, both storage modes, source updates, and retained keypath configuration.

Related issue

Follow-up to #334: value updates were fixed there; this addresses the remaining loss of wrapper identity. All-state searches found no existing PR for that behavior.

Validation

• Before the fix: three regression subcases failed; the two non-wrapping controls passed.
• Focused modules: 125 tests passed.
• Full Windows Python 3.12 suite: 885 tests run, 884 passed and 1 skipped; total coverage 98%.
• Mypy 2.3.1 passed for all 64 source files; all configured pre-commit hooks passed, including pinned Ruff 0.15.20 and mypy 2.1.0.
• Other OS/Python combinations remain for CI.

Checklist before requesting a review

• I have performed a self-review of my code (Codex review; no human review claimed).
• I have added tests for the proposed changes.
• I have run the tests and there are not errors.

AI disclosure: OpenAI Codex investigated the bug, prepared the patch and tests, reviewed the diff, ran verification, and drafted this description. No third-party implementation was copied.