Preserve in-place message on signing failure

[fegge]

Summary

• Preserve the caller's in-place message buffer when crypto_sign fails before producing a signature.
• Document the failure behavior for signed-message APIs.
• Add regression coverage for in-place signing failure and RNG-failure paths.

Validation

• clang-format -i mldsa/src/sign.c mldsa/src/sign.h mldsa/mldsa_native.h test/src/test_mldsa.c test/src/test_rng_fail.c: passed
• git diff --check -- mldsa/src/sign.c mldsa/src/sign.h mldsa/mldsa_native.h test/src/test_mldsa.c test/src/test_rng_fail.c: passed
• make run_func run_rng_fail -j4: passed
• Not run: ./scripts/format because nixpkgs-fmt is not installed in this local environment
• Not run: ./scripts/lint because shfmt is not installed in this local environment

Fixes #1217

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This work was completed by Trail of Bits as part of the Patch The Planet project in collaboration with OpenAI. The issue was identified primarily by the Codex coding agent, and manually reviewed before submission.

[fegge]

Reopened from an upstream repository branch with DCO-signed commits so full CI can run: #1230