Fix oracle: validate u128-to-i128 cast before using as price

[Uchechukwu-Ekezie]

Closes #383

Problem

get_stable_price and get_price_with_stable_fallback both cast a stored u128 directly to i128 without validation. A u128 value at or above 2^127 reinterprets as a negative i128 on this cast; that negative value:

• Passes the price==0 check (it isn't zero)
• Is returned unchanged as a legitimate price rather than rejected
• Downstream code assuming a stable price is always positive would silently misbehave

Fix

• get_stable_price: Validate raw u128 is below i128::MAX before casting. Reject if zero or would cast to negative, return None
• get_price_with_stable_fallback: Only use stable price if it's positive AND fits in i128. Otherwise fall back to primary price

This matches the validation applied to keeper-submitted prices which explicitly reject non-positive prices at write time.

Testing

• Oracle contract compiles without errors
• Validation logic prevents negative prices from being accepted
• Fallback to primary price when stable price is invalid

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[IbrahimIjai]

GReat...