fix: use %zu/size_t for message count in C example

[prql-bot]

Nightly survey finding. The minimal-c example printed the error count with %ld and iterated the message array with an int loop counter, but CompileResult.messages_len is a size_t.

On LP64 (Linux/macOS) long and size_t are both 64-bit, so this happens to work — which is why it went unnoticed. On LLP64 (Windows x64) long is 32-bit while size_t is 64-bit, so %ld is a width/signedness mismatch: passing a size_t where a long is expected is undefined behavior and can print a garbage error count. The int i < res.messages_len loop is also a signed/unsigned comparison.

Fix: use %zu and a size_t loop counter, both correct on every platform.

The example is copy-paste starter code for C consumers of prqlc-c, so getting the idiom right matters.

Verified with gcc -fsyntax-only -Wall -Wextra — compiles cleanly with no warnings (the previous version emitted a -Wformat and a -Wsign-compare).

No automated regression test: the minimal-c example is built by its own Makefile (not compiled in CI with -Werror), and the misbehavior only manifests at runtime on LLP64 targets, so there's no CI harness to assert against. The clean -Wall -Wextra compile above is the practical check.