fix(libcups3): adapt existing API to CUPS 3

[Abd002]

PR Description

This PR updates cups-rs so the existing API builds and works with libcups 3.

The change is limited to adapting the current wrappers to libcups 3 API/signature changes.

Changes

• Regenerate bindings against libcups 3.
• Update renamed CUPS constants and enum values.
• Adapt libcups boolean/count/index arguments to the CUPS 3 signatures.
• Replace removed CUPS media-size type usage with the current media type.
• Remove obsolete CUPS 2 compatibility checks.
• Keep the public API behavior focused on the existing crate functionality.

Requirements

This PR requires libcups 3. For local testing, libcups 3 can be built from OpenPrinting/libcups:

git clone --recurse-submodules https://github.com/OpenPrinting/libcups.git
cd libcups
./configure \
  --prefix=/usr/local \
  --with-domainsocket=/run/cups/cups.sock
make -j"$(nproc)"
sudo make install
sudo ldconfig

Testing

Tested with libcups 3:

cargo fmt --all -- --check
cargo check --all-targets
cargo test --all-targets --quiet

[Abd002]

I updated the branch so cups-rs can now build with either CUPS 2 or CUPS 3.

How it works:

• build.rs checks pkg-config for libcups.
• By default it tries cups3 first.
• If cups3 is not available, it falls back to the normal cups pkg-config package.
• It then sets either cfg(cups3) or cfg(cups2) for the Rust code.
• The version-specific C API differences are hidden in small compatibility paths, so the public Rust API stays the same.

For testing a specific version:

CUPS 3:
cargo check --all-targets --features force-cups3

CUPS 2:
cargo check --all-targets --features force-cups2

The force features are only for testing/packaging when we want to make sure a specific CUPS version is used. Normal users do not need to enable anything.

If no feature is added, the build still works automatically:

• uses CUPS 3 if cups3.pc exists
• otherwise uses CUPS 2 if cups.pc exists