feat(strategies): add strict interior-gap error to BacktestPriceFeed

[shobhamerabacha-star]

Summary

Adds an opt-in strict mode to BacktestPriceFeed that throws a new PriceGapError when a requested timestamp falls within the feed range but does not match any supplied price point.

Changes

• Added BacktestPricePoint, PriceGapError, and isPriceGapError to packages/strategies/src/types.ts.
• PriceGapError captures the requested timestamp and the bracketing points (before and after).
• Extended BacktestPriceFeed constructor and BacktestPriceFeed.create to accept an optional strict option, defaulting to false.
• In strict mode, reads matching no supplied point throw PriceGapError, while reads outside [first, last] still throw TimestampOutOfRangeError.
• Default carry-forward behavior is preserved untouched for backwards compatibility.
• Added test coverage for exact hits, interior gaps in both modes, and range boundary reads.

Verification

• Vitest unit tests in @meridian/strategies: 581 passed (100% green)
• TypeScript typecheck (tsc --noEmit): passed
• Soroban contracts suite: 100% passed

Closes #1015

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