Harden vNext snapshot numeric constructor and oversized integer errors

[Pigbibi]

Follow-up from PR #251 non-blocking review.

Scope:

• Make direct ValidatedPosition construction enforce the same text/numeric invariants as from_wire.
• Reject arbitrarily large Python/JSON integers with sanitized SessionContractError before float() can raise OverflowError.
• Add focused regression coverage.

Boundary: research-only contract hardening; no simulator algorithm, QPK store, live execution, orders, funds, or leverage changes.