fix(deps): make cryptography an optional test dependency

[AlexanderKomarov]

Changes in this pull request

cryptography is declared as a runtime dependency, but it is not used by the
runtime bindings in src/c2pa — all cryptographic operations (hashing, COSE
signing/verification, certificate-chain validation) happen inside the bundled
native c2pa-rs library. It is only imported by the test suite and the examples
to implement Signer.from_callback helpers (ES256 / ECDSA over PEM keys);
requirements.txt even noted it was "only used in the training example".

Declaring it under [project.dependencies] forces every downstream consumer to
install cryptography (~14 MB, bundles its own OpenSSL) with no runtime benefit,
enlarging install size and CVE/compliance surface.

This PR:

• moves cryptography to a test optional-dependency in pyproject.toml
  (mirroring requirements-dev.txt);
• drops it from requirements.txt (which already flagged it as example-only);
• installs the built wheel with the [test] extra in the wheel-test CI jobs so
  the suite still has it.

No runtime behaviour changes.

Verified locally: a wheel built from this branch installs c2pa-python,
toml, requests only (no cryptography), imports fine, and reads a real
signed C2PA asset; pip install "<wheel>[test]" pulls cryptography for the
tests.

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• Any TO DO items (or similar) have been entered as GitHub issues — N/A (no TODOs)