fix: make deeply-nested-body test platform-independent (fixes #3146)

[g0rdonL]

Fixes #3146.

Root cause

The test assumed a 100k-deep body always makes json.loads raise RecursionError. Since CPython 3.12 the C json scanner guards recursion by remaining C-stack headroom, not a fixed depth — so the outcome depends on the thread's stack size. Measured on Apple Silicon macOS, CPython 3.14:

• main thread: flips to RecursionError at depth ~74k → deep body fails to parse → PARSE_ERROR
• thread with a 64 MB stack: depth 100k parses fine → giant list fails request validation → INVALID_REQUEST

macOS runner threads get a smaller default stack than the main thread, which is why macos-latest sees -32600 while Ubuntu/Windows see -32700. Both are correct 400 rejections of the same hostile body; the old assertion was pinning a platform accident.

Change (test-only)

• test_modern_post_with_deeply_nested_body_is_rejected_not_a_crash: asserts 400 with either PARSE_ERROR or INVALID_REQUEST — the invariant that actually matters is "rejected, not crashed".
• New test_modern_post_recursion_error_during_parse_is_parse_error: monkeypatches the body parse to raise RecursionError, pinning the RecursionError → PARSE_ERROR mapping deterministically on every platform (this is what the old test was really trying to cover).

Verified locally on macOS arm64: both tests pass on CPython 3.13 and 3.14, full test_streamable_http_modern.py 52/52, ruff + pyright clean.

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