fix: Replace unsafe in-operator checks with compile-time type guards

[Who-is-PS]

Issue #, if available:

Description of changes:

TypeScript doesn't validate the in operator at compile time — 'banana' in obj compiles without error. This is the same class of bug that caused the COE (property filter not creating filtering tokens), where a 'prop' in obj check silently passed with an incorrect property name.

All 3 source-file usages in this repo discriminate the PropertyFilterProps.Token | PropertyFilterProps.TokenGroup union — the exact same type that caused the original bug.

Solution

Replace all raw 'prop' in obj checks with type guard functions that use keyof to validate property names at compile time. If someone typos a property name, TypeScript will now surface a compile error instead of silently allowing it.

Changes

• New: src/common/property-filter-type-guards.ts — shared isTokenGroup() and isToken() type guards
• Updated: src/pages/table-saved-filters/filter-set-modals.tsx — use isTokenGroup()
• Updated: src/fake-server/distributions.ts — use isTokenGroup() and isToken()
• Updated: test/e2e/a11y.test.ts — local isAxeError() type guard

4 checks replaced across 3 files. Zero behavioral changes.

Verification

• No visual or behavioral changes
• All existing tests pass

References

• COE: Property filter not creating filtering tokens
• SIM: AWSUI-59006
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• Tech Proposal. (k9wvA3eNMiZ9).