Add macos-latest to the CI test matrix #3597

Description

The macOS-only test failure reported in #3146 went unnoticed for an extended period because the CI matrix runs Ubuntu and Windows only. The failing test (test_modern_post_with_deeply_nested_body_is_rejected_not_a_crash) encodes an assumption about where CPython's recursion guard strikes, and that assumption silently broke on macOS + Python 3.14 — a platform/interpreter combination the matrix never exercised.

Adding macos-latest to the test matrix would have surfaced this at the point it was introduced rather than via a downstream bug report. The failure was deterministic and reproducible across repeated macos-latest runs, so it is a reliable signal, not flaky-runner noise.

Worth noting for scoping: the failure mode is CPython-version-sensitive (pre-3.14 the recursion limit ignores stack size entirely, so the test is deterministic and platform-independent there; on 3.14 the outcome depends on per-thread stack headroom). A macOS matrix leg therefore provides the most value on 3.14+, but including it across the supported range also guards against other platform-specific regressions in the server transport stack.

Context: this was flagged during review of #3147 (which fixes #3146) as the "more valuable half" of that issue — splitting it out so it doesn't get closed along with the PR.

References

• Deeply-nested-body test fails on macOS: rejected as INVALID_REQUEST (-32600) instead of PARSE_ERROR (-32700) #3146 — the macOS failure this leg would have caught
• fix: make deeply-nested-body test platform-independent (fixes #3146) #3147 — fix for Deeply-nested-body test fails on macOS: rejected as INVALID_REQUEST (-32600) instead of PARSE_ERROR (-32700) #3146, review discussion where the matrix gap was raised

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