fix: reject a zero-scaled feed price and staleness-check the composite USD leg

[nikkaroraa]

Addresses two Bugbot findings on the Aave v4 price feeds (raised on #199). Both are behavior-preserving hardening; also pushed to feat/lend-price-feeds so #199/master stays in sync.

Context

Neither issue was introduced by the recent BaseAaveV4PriceFeed extraction — both existed verbatim in the original feeds. The extraction just centralized the logic into _composeUsd, which is now the single place to fix them. The production listing script (AddSummerLendCollateral) already wires the safe pattern; the unsafe wiring only lived in the fork tests.

Fixes

• Stale USD leg (High): the composite only staleness-checked the rate leg; the USD leg's freshness was trusted. _composeUsd can't check the underlying's age (the IAaveV4PriceFeed interface has no timestamp), so the invariant is now documented in NatSpec — the underlying must be a staleness-checking feed, never a raw aggregator. The fork tests were rewired to wrap ETH/USD in a ChainlinkPriceFeed (the pattern prod already uses), and a new test_reverts_whenUnderlyingStale proves a stale underlying makes the composite revert StalePrice.
• Zero scaled price (Medium): a positive rate could floor to zero after decimal scaling and be returned as a valid 0. _composeUsd now reverts InvalidPrice on a zero result (matching PriceProviderV2). New test_latestAnswer_revertsWhenScaledPriceFloorsToZero covers it.

Testing

forge build clean, forge fmt/forge lint clean. All 33 feed tests pass (test/price-provider/{Chainlink,VedaAccountant,Pyth}PriceFeed.t.sol), including the 3 new ones.

Follow-up (not in this PR)

scripts/aave-v4/DeployAaveV4TestInstance.s.sol (a dev test-instance deployer, not prod) still wires a raw ETH/USD aggregator as the underlying. Worth aligning to the wrapped pattern for consistency, but it's dev-only and doesn't affect deployed feeds.

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Note

Medium Risk
Changes oracle collateral pricing invariants (no zero prices, composite staleness via wiring); behavior-preserving hardening but affects lending risk if misconfigured underlying feeds remain in production.

Overview
Hardens Aave v4 composite price feeds in BaseAaveV4PriceFeed._composeUsd so collateral prices cannot be reported as 0 after decimal scaling, and documents how the USD leg must be wired for freshness.

_composeUsd now uses a single path through stable snapping, then reverts InvalidPrice if the result is zero (e.g. a positive rate that floors away in integer division). NatSpec also states that underlyingUsdFeed must be a staleness-checking IAaveV4PriceFeed, not a raw Chainlink aggregator, because the base trusts latestAnswer() and does not re-check timestamps on that leg.

Fork tests for Chainlink and VedaAccountant composites were updated to wrap ETH/USD in a ChainlinkPriceFeed (matching production wiring), with new test_reverts_whenUnderlyingStale cases. Pyth adds test_latestAnswer_revertsWhenScaledPriceFloorsToZero for the zero-after-scaling case.

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