feat(pathRouter): add findPath with configurable fallback threshold

[kingsrichie79-cmyk]

Closes #699

Description:
This PR resolves an issue where src/pathRouter.ts would return a hard failure (PathNotFoundError) when no payment paths met the required minimum liquidity threshold on the first attempt.

To provide callers with a viable (even if more costly) route, this update implements a single-retry fallback mechanism that can dynamically relax the cost threshold.

Key Changes & Implementations:

• Introduced a new FindPathParams interface encompassing sourceAsset, destinationAsset, threshold, and fallbackSlippagePct.
• Added a new findPath(params) method to the PathRouter class.
• Configured a fallback check that intercepts empty query results: if no path meets the initial query, it automatically retries with a relaxed threshold computed as threshold * (1 + fallbackSlippagePct/100).
• Modified PathResult to include an optional usedFallback: true telemetry flag to let callers know when the fallback mechanism was utilized to yield the path.
• Ensured this fallback retry occurs exactly once to prevent unbounded multi-level chaining.

Acceptance Criteria Met:

• findPath(params) accepts a fallbackSlippagePct?: number option.
• When no path meets the primary threshold, the router retries with threshold * (1 + fallbackSlippagePct/100).
• The returned path from the fallback query includes the usedFallback: true flag.

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