Fix false positive bad-override when overriding methods using bounded TypeVars

[kavix]

Summary

Fixes #4847

When checking override compatibility between generic methods, type variables in the child method signature are instantiated with fresh inference variables (v_T) and matched contravariantly against the parent method's parameters.

When a parameter is a union like T | None, is_subset_eq_no_recursive_check previously attempted to check whether the TypeVar's bound was a subtype of the union before splitting the RHS union. When the RHS union contained an unsolved inference variable, the speculative check succeeded under with_snapshot and rolled back all variable assignments, leaving the child method's type variable unconstrained and causing override validation to fail with a false-positive bad-override.

This PR:

1. Removes the speculative pre-union Type::Quantified checks in is_subset_eq_no_recursive_check.
2. In (l, Type::Union(u_union)), when checking whether a Type::Quantified is a subtype of a union, each member of the RHS union is checked first. If none match, a fallback verifies whether the TypeVar's bound or constraints as a whole satisfy the union (preserving the intended behavior for constrained and union-bounded type parameters).
3. Unifies post-union (Type::Quantified(q), u) resolution across Restriction::Bound, Restriction::Constraints, and Restriction::Unrestricted.

Test Plan

• Added integration tests covering module-level bounded TypeVars, constrained TypeVars, and PEP 695 syntax in pyrefly/lib/test/class_overrides.rs.
• cargo test test::class_overrides
• cargo test test::generic_restrictions
• cargo test test_generic
• python3 test.py --no-test --no-tensor-shapes --no-conformance --no-jsonschema

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

============================================================
SUMMARY
============================================================
Total: +4 new errors, -10 fixed errors
By preset: +3/-6 (default), +3/-6 (strict)

Projects with changes (6):
  altair: +0 -1
  scikit-build-core: +2 -1
  starlette: +0 -1
  pytest-autoprofile: +1 -3
  core: +1 -3
  artigraph: +0 -1
============================================================

FULL DIFF DETAILS
------------------------------------------------------------

altair (https://github.com/vega/altair)
- ERROR altair/vegalite/v6/api.py:1088:43-63: The type of this argument is unknown [unknown-argument-type]

scikit-build-core (https://github.com/scikit-build/scikit-build-core)
- ERROR src/scikit_build_core/metadata/__init__.py:145:16-148:10: Returned type `dict[Unknown, dict[str, str]]` is not assignable to declared return type `T` [bad-return]
+ ERROR src/scikit_build_core/metadata/__init__.py:145:16-148:10: Returned type `dict[str, dict[str, str]]` is not assignable to declared return type `T` [bad-return]
+ ERROR src/scikit_build_core/metadata/__init__.py:146:64-65: Unused `# type: ignore` comment [unused-type-ignore]

starlette (https://github.com/encode/starlette)
- ERROR starlette/concurrency.py:34:43-47: Argument `(ParamSpec(P)) -> T` is not assignable to parameter `func` with type `(**tuple[*@_]) -> @_` in function `anyio.to_thread.run_sync` [bad-argument-type]

pytest-autoprofile (https://gitlab.com/TTsangSC/pytest-autoprofile)
- ERROR tests/test-modules-and-tests/test_subprocess/packages/myprocess.py:89:25-31: `(ParamSpec(PS)) -> T` is not assignable to attribute `func` with type `() -> T` [bad-assignment]
- ERROR src/pytest_autoprofile/_patches.py:721:16-28: Returned type `_ExtraArgWrapper[(ParamSpec(PS)) -> T, SerializedStash] | _ExtraArgWrapper[(ParamSpec(PS)) -> T, AutoProfStash] | _PureWrapper[(ParamSpec(PS)) -> T]` is not assignable to declared return type `_ExtraArgWrapper[Any, SerializedStash] | _ExtraArgWrapper[Any, AutoProfStash] | _PureWrapper[Any]` [bad-return]
+ ERROR src/pytest_autoprofile/_patches.py:721:16-28: Returned type `_PureWrapper[(ParamSpec(PS)) -> T]` is not assignable to declared return type `_ExtraArgWrapper[Any, SerializedStash] | _ExtraArgWrapper[Any, AutoProfStash] | _PureWrapper[Any]` [bad-return]
- ERROR src/pytest_autoprofile/importers.py:1656:16-20: Returned type `(ParamSpec(PS)) -> T` is not assignable to declared return type `() -> T` [bad-return]

core (https://github.com/home-assistant/core)
- ERROR homeassistant/components/asuswrt/helpers.py:42:16-70: Returned type `dict[str | Unknown, Unknown]` is not assignable to declared return type `T` [bad-return]
+ ERROR homeassistant/components/asuswrt/helpers.py:42:16-70: Returned type `dict[str, Any]` is not assignable to declared return type `T` [bad-return]
- ERROR homeassistant/components/asuswrt/helpers.py:42:37-38: The type of this argument is unknown [unknown-argument-type]
- ERROR homeassistant/components/asuswrt/helpers.py:42:40-41: The type of this argument is unknown [unknown-argument-type]

artigraph (https://github.com/artigraph/artigraph)
- ERROR src/arti/internal/type_hints.py:172:46-53: The type of this argument is unknown [unknown-argument-type]