Preserve runtime types and bounded fresh-string fallback semantics

[CaelmBleidd]

Unknown-call fallback lost finite return alternatives and sometimes treated ordinary object fields as builtin storage. Preserve finite literal return kinds, null/undefined and intersections; resolve inherited fields and ordinary object.length using the actual type contract. Fresh string mocks expose bounded UTF-16 backing rather than erasing strings to an unconstrained address. Synthetic EtsFakeType wrappers remain internal storage metadata and are rejected in type constraints before subtype shortcuts.

JacoDB is pinned to merged neo revision 7596ae5834, which includes JacoDB #434. Its frontend fix preserves explicitly declared null/undefined alternatives in call return types when TypeScript strictNullChecks is disabled.

Review: finite-union fallback in TsMock/TsExprResolver; structural/runtime field selection; the fake-type constraint invariant; fresh string bounds and source regressions; emitted runtime limitations. Open unions keep generic fallback and bounded-string capacity remains an explicit limit.

Local validation for f4be53969 with the published JacoDB artifact and its bundled TypeScript frontend: 65 tests passed, with 0 failures and 4 existing disabled tests skipped across type-system contracts, fresh-string returns, property presence, field access, typeof and truthiness/replay. The finite-union source assertion retains all five return alternatives, including null and undefined. The new invariant test fails on the previous implementation because no IllegalArgumentException is thrown, and passes with the restored require. Production/test compilation, Detekt main/test (0 code smells) and git diff --check passed.