Summary

@contextmanager is slow, and it's used in hot paths for every request both in httpcore2 and httpx2.

This PR replaces uses of map_exceptions and map_httpcore_exceptions with faster alternatives.

On my machine, a pytest-benchmark test shows a nearly 10% speed increase on a microbenchmark that retrieves a file from a local Caddy (see source in the details block below)

----------------- benchmark: 2 tests, 2 sources ------------------
Name (time in ms)     0010_82b9e2d Min  0042_384ae31 Min      ΔMin
------------------------------------------------------------------
test_connpool                   3.8101            3.4641     -9.1%
test_client                     4.0599            3.7075     -8.7%
------------------------------------------------------------------

import pytest
from httpx2 import Client
from httpcore2 import ConnectionPool


def test_connpool(benchmark):
    with ConnectionPool() as pool:
        def b():
            resp = pool.request(
                method="GET",
                url="http://127.0.0.1:9339/bigfile2",
            )
            assert len(resp.content) == 10 * 1024 * 1024
        benchmark(b)


def test_client(benchmark):
    with Client() as c:
        def b():
            resp = c.request(
                method="GET",
                url="http://127.0.0.1:9339/bigfile2",
            )
            assert len(resp.content) == 10 * 1024 * 1024
        benchmark(b)

and a hyperfine check for the same says ~5%, but that's with all of the overhead:

$ hyperfine --warmup=5 --min-runs=20 'git checkout main && uv sync && uv pip install ../pytest-benchmark && pytest --benchmark-autosave test_bench1.py' 'git checkout retire-map-exceptions-hx2 && uv sync && uv pip install ../pytest-benchmark && pytest --benchmark-autosave test_bench1.py'
[...]
Summary
  git checkout retire-map-exceptions-hx2 && uv sync && uv pip install ../pytest-benchmark && pytest --benchmark-autosave test_bench1.py ran
    1.05 ± 0.06 times faster than git checkout main && uv sync && uv pip install ../pytest-benchmark && pytest --benchmark-autosave test_bench1.py

Rebase of

• https://github.com/encode/httpx/pull/3778
• https://github.com/encode/httpcore/pull/1044

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
  • No test changes needed. Some pragma: no covers were added to the exception branches that were actually not covered originally either.
• I've updated the documentation accordingly.
  • No public documentation changes needed, I think.