[Contract] Audit and harden `flash_loan_guard` against oracle freshness and sequencing attacks

[Solomonsolomonsolomon]

Extend the current price-snapshot model to handle stale or manipulated oracle data, delayed updates, and edge conditions around ledger timing. The goal is to make the guard defensible under adversarial market conditions, not just basic happy-path checks.

[king-aj-the-first]

@king-aj-the-first has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hii, i'd like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @king-aj-the-first to this issue.