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 94.50%. Comparing base (80a5974) to head (b088294).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #742   +/-   ##
=======================================
  Coverage   94.50%   94.50%           
=======================================
  Files          48       48           
  Lines        4916     4916           
=======================================
  Hits         4646     4646           
  Misses        270      270           

Flag	Coverage Δ
integration	90.66% <ø> (-0.17%)	⬇️
unit	83.03% <ø> (ø)

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