fix: throw when MessagePack size exceeds uint32 limit (#5320)

[ameliabarnabyhub]

Fixes #5320

to_msgpack() now throws out_of_range.412 when a string, array, binary, or object reports a size above UINT32_MAX, matching the documented MessagePack limit and the BSON fix pattern from #5314.

Adds a regression test using a container type that reports an oversized size() without allocating multi-gigabyte data.

[gregmarr]

Why do you need this test when this is only called when this is known to be true?

[gregmarr]

Since this is always passed a pointer, don't need the default argument.

[github-actions]

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[nlohmann]

Critical: this doesn't compile. tests/src/unit-msgpack.cpp line ~1991 — the new test does CHECK_THROWS_WITH_AS(_ = huge_json::to_msgpack(j), ...), but unlike every other use of this discard-variable pattern in the file, no json _; is declared in this TEST_CASE. Compiling a minimal repro against the PR's headers reproduces:

error: use of undeclared identifier '_'

This matches what's happening on the PR itself — every CI check is currently failing (gcc, clang, MSVC, macOS, sanitizers, fuzzing, etc.), all traceable to this one build break. Fix: add json _; (or reuse the pattern from the other test cases in this file) before the CHECK_THROWS_WITH_AS call.

Minor: test coverage gap. The fix in binary_writer.hpp adds the same assert_msgpack_size guard to four independent branches (string, array, binary, object), but the new test only exercises the array case. Consider adding cases for the other three so a future regression in one branch doesn't slip through untested.

The core fix logic itself looks correct — it throws out_of_range.412 exactly when the size exceeds UINT32_MAX, matching the existing BSON pattern (to_bson_length). I verified against develop that oversized values previously fell through silently with no length byte written at all (malformed MessagePack output), so this does fix a real bug — it just needs the test to actually compile.

Written by Claude Code on behalf of @nlohmann.