test(regression): #915 — standalone + Fastify async-route jwt.sign tests

[andrewtdiz]

Summary

The original codegen fix for #915 landed via PR #920 (fix(codegen): #915 — jwt.sign(...) dispatch calling-convention mismatch SIGSEGVed after async resume, commit ccf96c26) which took a more general approach using NativeArgKind::JsonStringify lowering.

This PR has been rebased to keep only the two regression tests authored here, since they strengthen the regression net beyond what #920's test covers:

• test-files/test_issue_915_jwt_sign.ts — pins the basic jwt.sign({sub:'x'}, 'secret') + string-payload shape standalone (no async, no Fastify).
• test-files/test_issue_915_jwt_sign_after_async_route.ts — pins the Fastify async-route shape (nested async function before jwt.sign), matching the original Native server SIGSEGV at jwt.sign(...) (jsonwebtoken) after resumed async-step body — follow-up to #859 #915 minimal repro.

Verification

• Standalone test compiles + runs against current main: object token typeof: string, string token typeof: string, exit 0.
• Fastify test requires fastify + jsonwebtoken installed; covered by existing test infrastructure.
• Cherry-picked from this PR's original commit, then dropped the now-redundant codegen changes (since fix(codegen): #915 — jwt.sign(...) NATIVE_MODULE_TABLE calling-convention SIGSEGV #920 fixes the underlying bug with a more general dispatch).

Closes #915 (test coverage half — codegen half closed by #920).

Note to author

Thank you @andrewtdiz — your diagnosis caught the same calling-convention mismatch our fix-agent found independently. Keeping your two test files as additional pinning since they cover shapes our regression doesn't.