refactor(checks)!: drop LesserThan and LesserThanEquals classes

[kevinmessiaen]

Closes ENG-1677

What

The Giskard Hub v3 now emits the correct LessThan / LessThanEquals names, so the misspelled alias classes have no remaining producer. This removes them from the public API while keeping their serialized kind strings loadable.

Why not a hard drop

LesserThan / LesserThanEquals were not just Python aliases — they were registered in the discriminated-union registry via @Check.register("lesser_than"). Deleting them outright would make any check JSON persisted before the Hub upgrade fail to deserialize with a hard ValidationError, not a degraded read.

So the classes are gone, but the legacy kinds still load:

@Check.register("less_than", aliases=["lesser_than"])
class LessThan(...): ...

aliases= registers extra discriminators on the read path only. Only the canonical kind is mapped back from the class, so a legacy payload loads as a real LessThan and is re-saved as less_than — migrating itself on the next write.

This lives in the shared Discriminated registry rather than locally in giskard-checks because __get_pydantic_core_schema__ installs a plain validator that replaces the model schema. Validation dispatches to the concrete class before any Check-level model_validator could rewrite kind, so a downstream fix would have required monkeypatching registry internals.

Registry fixes found while in there

• Atomic registration. Validating and writing in a single pass left the registry half-populated when a later alias collided — a class whose decorator raised stayed loadable. Now validate-then-write.
• Dropped _reverse_kinds, which held data byte-identical to _subclasses (verified across all 7 registered bases). Reading _subclasses via .get() also stops the former defaultdict from inserting an empty entry on each failed lookup.
• Reject a bare str for aliases. aliases="lesser_than" would silently register one kind per character; str satisfies Sequence[str], so no type checker catches it. aliases is now keyword-only.

Breaking change

LesserThan and LesserThanEquals are no longer importable from giskard.checks — use LessThan / LessThanEquals. Stored payloads using the legacy kind strings are unaffected. Worth a release-note line for OSS 3.0.

Testing

• giskard-core 47 passed, giskard-checks 804 passed / 4 skipped
• make check exit 0 (basedpyright 0 errors, pip-audit + licensecheck clean)
• The atomicity test was verified to actually catch the bug — it fails with DID NOT RAISE against the single-pass version
• giskard-agents has 7 pre-existing failures from a missing Gemini API key; confirmed identical on a clean tree

Follow-up

Legacy alias hits are not observable: _telemetry_props.py reads kind off the instance, which now always returns the canonical name. Without a counter or debug log at the alias-match site, these aliases can never be retired on evidence. Worth a separate ticket.

🤖 Generated with Claude Code

[linear-code]

ENG-1677

[henchaves]

LGTM, only need to update method names in test_comparison.py

[henchaves]

there are many lesser_than in method names within this file