Add ClusteringInitMethod enum constants to __init__.pyi stub

[alibeklfc]

Summary:
Add the three ClusteringInitMethod enum constants
(ClusteringInitMethod_RANDOM, ClusteringInitMethod_KMEANS_PLUS_PLUS,
ClusteringInitMethod_AFK_MC2) and the ClusteringInitMethod type alias to
the authoritative init.pyi Pyre stub.

These constants were introduced in commit bc5fe3d75e85 (Dec 2025) when
k-means++ and AFK-MC² centroid initialization were added to Clustering.h.
They are accessible at runtime via the SWIG-generated module but were never
added to init.pyi. When D107274305 (Jun 2026) wired init.pyi as
the authoritative Pyre source, all code using these constants acquired
[missing-attribute] errors. test_clustering_initialization.py already
references them 14 times.

Fix: add ClusteringInitMethod = int type alias and three int constants
near the MetricType block at the top of the stub (matching the established
pattern for SWIG-wrapped scoped enums). Update ClusteringParameters.init_method
from 'int # comment' to 'ClusteringInitMethod' to match how MetricType
is used throughout the file.

Test: buck test fbcode//faiss/tests:test_swig_wrapper — 25 pass, 0 fail
(24 pre-existing + 1 new: test_clustering_init_method_enum_values).
Test session: https://www.internalfb.com/intern/testinfra/testrun/32369622335973550

Reviewed By: limqiying

Differential Revision: D109007472

[meta-codesync]

@alibeklfc has exported this pull request. If you are a Meta employee, you can view the originating Diff in D109007472.