Ensure we strip NUL bytes before CString::new

[Shaptic]

What

Handle NulError from CString::new and strip \0 bytes.

Why

While our usage is safe because the strings come from the WASM host VM which never includes user-controlled messages and only uses printable ASCII characters, it's always better to code defensively.

Known limitations

n/a

[Copilot]

Pull request overview

This PR hardens Rust↔C FFI string handling by ensuring interior NUL (\0) bytes can’t cause CString::new() to panic, centralizing the behavior in a shared helper and updating call sites to use it.

Changes:

• Added safe_cstring() helper in the ffi crate to strip interior NUL bytes on CString::new() failure.
• Updated string_to_c() (and preflight error-string construction) to use safe_cstring() instead of CString::new(...).unwrap().
• Removed a duplicated local string_to_c() helper from the preflight crate.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
cmd/stellar-rpc/lib/preflight/src/lib.rs	Switches preflight error-string construction to safe_cstring() and removes the local string_to_c() helper.
cmd/stellar-rpc/lib/ffi/src/lib.rs	Introduces safe_cstring() and routes string_to_c() through it to avoid panics on interior NUL bytes.

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