fix: preserve integer enum and const values in type errors

[Kaileshwar16]

Resolves #287

Schemas combining type: "integer" with integer enum or const values incorrectly reported
“A value is not allowed here” when validation failed. For example, enum: [1, 2] now
correctly reports “Expected one of 1 or 2”.

Use a shared type-compatibility predicate for enum and const filtering, checking
serialized numeric values with Number.isInteger. Preserve existing number/string
behavior, message formatting, and schema-location attribution.

Add 14 regression fixtures covering integers, fractions, union types, negative values,
zero, exponential notation, incompatible types, successful validation, and nested
locations through both public API paths.

Validation:

• Before fix: 29 focused failures.
• After fix: 67 focused tests passed.
• Full suite: 1,313 tests passed.
• Lint and type check passed.

Environment note: Vitest warned that Node 25 is outside its supported engine range; all
checks completed successfully.