fix(q): distinguish decode failures from Q errors

[belowzeroff]

Summary

• Return NULL with a decode reason for malformed or unsupported wire data, matching the documented q_decode() contract.
• Preserve RAY_ERROR for a valid Q error frame.
• Add codec regressions for unsupported wire types and malformed Q error frames, and align malformed-frame assertions with the documented API.

User-facing behavior

Callers can distinguish a remote Q error from a local decode failure without inspecting or guessing from an error object.

Tests

• Built the test driver against the available Rayforce core library.
• Ran --codec-selftest: passed.
• git diff --check: passed.

This is stacked on PR RayforceDB#15 and targets fix/q-codec-transport-safety in this fork so the PR contains only this follow-up change.