Detailed steps to reproduce the problem?

Include full traceback, if possible

cryptography 50.0.0 added MLKEM768PublicKey and MLKEM1024PublicKey to CertificatePublicKeyTypes, the return type of Certificate.public_key(). pymongo.ocsp_support._verify_signature dispatches on key type and falls through to a generic key.verify(signature, data) branch, but ML-KEM is a key encapsulation mechanism and its public keys have no verify() method.

>>> from cryptography.hazmat.primitives.asymmetric import mlkem
>>> from pymongo.ocsp_support import _verify_signature
>>> key = mlkem.MLKEM768PrivateKey.generate().public_key()
>>> _verify_signature(key, b"sig", None, b"data")
AttributeError: 'cryptography.hazmat.bindings._rust.openssl.mlkem.MLKEM768PublicKey' object has no attribute 'verify'

The same drift also breaks type checking. ocsp_support hand-maintains a CertificateIssuerPublicKeyTypes union because cryptography exports no public alias for it, and that union is now narrower than what public_key() returns:

pymongo/ocsp_support.py:236: error: Argument 1 to "_verify_signature" has incompatible type
  "DSAPublicKey | RSAPublicKey | EllipticCurvePublicKey | Ed25519PublicKey | Ed448PublicKey | <7 more items>";
  expected "DSAPublicKey | ... | MLDSA44PublicKey | MLDSA65PublicKey | MLDSA87PublicKey | X25519PublicKey | X448PublicKey"  [arg-type]
pymongo/ocsp_support.py:245: error: Argument 1 to "_verify_signature" has incompatible type ... [arg-type]

This currently fails the static and Typing Tests CI jobs on any branch whose typing environment resolves cryptography 50.

An earlier instance of the same drift was fixed in PYTHON-5915, which added the ML-DSA types when the minimum was raised to cryptography 47.

Definition of done: what must be done to consider the task complete?

• _verify_signature handles ML-KEM public keys without raising. It should fail closed and return 0: an ML-KEM key cannot produce a signature, so an OCSP signature can never be verified against one.
• The CertificateIssuerPublicKeyTypes union matches cryptography's CertificatePublicKeyTypes, so mypy passes on both the minimum supported cryptography (47.0.0) and current (50.0.0).
• Regression tests covering both ML-KEM key sizes.

The exact Python version used, with patch level:

3.14.2 (main, Dec 9 2025, 19:29:30) [Clang 21.1.4]

The exact version of PyMongo used, with patch level:

4.18.0.dev0, has_c() is True

Describe how MongoDB is set up. Local vs Hosted, version, topology, load balanced, etc.

Not applicable. The defect is in certificate handling and reproduces without a server; it is triggered by the installed cryptography version, not by any deployment.

The operating system and version (e.g. Windows 7, OSX 10.8, ...)

macOS 26.6.1. Not platform specific; the CI failures are on Linux.

Web framework or asynchronous network library used, if any, with version (e.g. Django 1.7, mod_wsgi 4.3.0, gevent 1.0.1, Tornado 4.0.2, ...)

None.