refactor: Drop the httpx2 import alias and correct HTTPX2 naming

[vdusek]

_httpx2.py pulled the library in as import httpx2 as httpx, so the module and its tests read
httpx.* throughout while the installed package is httpx2. Dropping the alias lets the code name
the library it actually uses.

The rename follows through: the private transport attributes become _httpx2_client and
_httpx2_async_client, matching the sibling adapter's _impit_client / _impit_async_client, and
21 test functions plus the UNSET_HTTPX2_TIMEOUT constant lose the old spelling. All of it is
private, so no public name moves. A search for the original httpx under src/ and tests/ now
turns up only the two sentences that describe the library's lineage on purpose.

It also cleans up link labels left over from #1048. Eight places wrote
[HTTPX](https://github.com/pydantic/httpx2), giving the current client the old library's name, and
the surrounding prose had the same split: docstrings said HTTPX while the link beside them said
HTTPX2. The library calls itself HTTPX2, so 66 prose references now use that name. The v2 upgrade
guide is the reverse case: it describes moving off the original httpx, so its link goes back to
python-httpx.org, matching what the 2.5 docs already say. These changes land in docs/ only, so
they reach the docs site at /next until the next version snapshot.

No behavior changes. The 241 unit tests covering the HTTP clients, headers, timeouts, and the
pluggable client still pass, 21 of them under new names.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.24%. Comparing base (3e6f1ea) to head (6143d11).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1052   +/-   ##
=======================================
  Coverage   95.24%   95.24%           
=======================================
  Files          59       59           
  Lines        5509     5509           
=======================================
  Hits         5247     5247           
  Misses        262      262           

Flag	Coverage Δ
integration	92.17% <78.94%> (+0.12%)	⬆️
unit	87.34% <100.00%> (ø)

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