fix zero-length nanoids and update pgrx to 0.19.2

[Vonng]

idkit_nanoid_custom_generate_text(0, 'abc') enters a non-terminating loop in nanoid 0.4.0. Return an empty string when the normalized length is zero, preserving the existing negative-length clamp and positive-length generation. Add regression tests for zero, -1, and i64::MIN.

Fixes #184

This change is based on current upstream main (3183860) and preserves its dependency and CI action updates. The lockfile resolves uuid to 1.26.0 because pgrx 0.19.2 requires ^1.23.2; this remains within the project's existing ^1.20 requirement. The upstream getrandom 0.4.1 and recent anyhow, chrono, time, and keccak updates are retained.

The pgrx migration also:

• Pins pgrx and the existing pgrx-tests dependency to =0.19.2, refreshing their runtime and test dependency graph without adding a direct runtime dependency.
• Raises the declared MSRV to 1.96, pins Rust 1.98.0, and aligns current CI, Justfile, and Docker build defaults while retaining the existing OS base images.
• Removes the PG13 feature and RPM variant, retaining PG14–18.
• Removes the obsolete pgrx_embed binary declaration and source file, which use a macro removed from current pgrx.

Validation on Rocky Linux 9.8, aarch64, PostgreSQL 17.11, Rust 1.98.0, and cargo-pgrx 0.19.2:

• Release RPM build passed; main, debuginfo, and debugsource payloads were checked, including DWARF and runtime dependencies.
• Five targeted NanoID native tests passed; 39 other tests were filtered out.
• After reinstalling the produced RPM, SQL checks passed for NanoID boundaries, valid lengths, alphabets, strict NULLs, and table defaults; UUIDv6/v7 text and UUID APIs plus timestamp extraction; and ULID/CUID2 smoke tests. The installed library matched the RPM payload by SHA-256.

The MSRV toolchain, other PG/OS combinations, and upstream CI were not run in this validation.

[t3hmrman]

LGTM 🚀

Sorry I'm a bit late here but thanks for the contribution @Vonng !