docs: clean up unreleased changelog section for v3.0.0

[vdusek]

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 (test: guard integration test resource names against 63-char API limit #729) — not user-facing.
• Drop the [**breaking**] marker on the pluggable HTTP client entry (feat: Make HTTP client pluggable with abstract base classes #641) — additive at the public API surface; default impit behavior unchanged.
• Move "Drop support for Python 3.10" (chore!: Drop support for Python 3.10 #636) to ⚙️ Miscellaneous Tasks, matching the precedent of prior version-drop entries.
• Move "Introduce fully typed clients" (feat!: Introduce fully typed clients #604) to 🚀 Features as the v3 headliner — closes the long-standing Write integration tests #21 / Introduce fully typed Python API client #481 type-annotation requests.
• Order breaking changes first within each section.

Important

Must be merged after the v3.0.0 release — otherwise CI (git-cliff) will regenerate the unreleased section and overwrite these edits.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.29%. Comparing base (76e4de4) to head (6f24106).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #742      +/-   ##
==========================================
- Coverage   95.38%   95.29%   -0.10%     
==========================================
  Files          45       45              
  Lines        5118     5118              
==========================================
- Hits         4882     4877       -5     
- Misses        236      241       +5     

Flag	Coverage Δ
integration	95.29% <ø> (-0.10%)	⬇️

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