fix(ffi): return Box<PickyError> from Pkcs12CryptoContext constructor

[Richard Markiewicz (thenextman)]

Pfx.FromDer (and anything that uses a Pkcs12CryptoContext) crashes or hangs from the .NET bindings. This was shipped in NuGet 2026.6.29; but I've since yanked the package from nuget.org because of this problem.

An FFI ABI mismatch was introduced in #448. That PR made the RNG-seeding fallible, so Pkcs12CryptoContext::with_password / no_password changed from infallible (Box<Pkcs12CryptoContext>) to Result<Box<Pkcs12CryptoContext>, Pkcs12Error>, but the generated C# bindings were not regenerated.

As a result the C# side still called these constructors with the old infallible ABI, using the returned pointer directly as the Pkcs12CryptoContext* handle. On the Rust side the function now returns a Diplomat Result wrapper pointer, so C# handed the wrapper to Pfx.FromDer; crypto_context.0.lock() then dereferenced the wrong pointer and wrote the mutex state out of bounds and corrupted the heap.

Two details made this easy to miss:

• Only the constructors were stale. Pfx.FromDer's own binding was ResultBox-aware, so the two were inconsistent.
• The constructors used a bare Pkcs12Error rather than the Box<PickyError> opaque error that every other Result-returning FFI function uses.

Fix

• Return Result<Box<Pkcs12CryptoContext>, Box<PickyError>> from both constructors, matching the rest of the FFI surface.
• Regenerate the Diplomat C# bindings. The wrappers now unwrap the ResultBox, throw PickyException on error, and pass the correct handle.