docs: clean up unreleased changelog section for v3.0.0 (#742)

## Summary

Clean up the unreleased changelog section for the v3.0.0 release:

- Rename header `2.5.1` → `3.0.0` and add the upgrade-guide link.
- Drop the test-only fix entry (#729) — not user-facing.
- Drop the `[**breaking**]` marker on the pluggable HTTP client entry
(#641) — additive at the public API surface; default impit behavior
unchanged.
- Move "Drop support for Python 3.10" (#636) to ⚙️ Miscellaneous Tasks,
matching the precedent of prior version-drop entries.
- Move "Introduce fully typed clients" (#604) to 🚀 Features as the v3
headliner — closes the long-standing #21 / #481 type-annotation
requests.
- Order breaking changes first within each section.