feat: expose source-chain principal refund recipients

[CallumGrindle]

Which Linear task is linked to this PR?

CORE-1248 — Allow passing different refund address than sender address

Why was it implemented this way?

• Expose optional refundAddress on RouteOptions, both contract-calls amount variants through their shared request type, and the deprecated single-call request.
• Document the existing quote/action fields consistently: source-chain bridge principal refunds only, separate from funding and successful recipients; destination swap/call recovery is unchanged. Unsupported bridges/quote variants must reject or be excluded, not silently fall back.
• Publish 18.6.0-beta.0 using the existing release:beta script and tag-triggered workflow. Mark alpha/beta GitHub releases as prereleases and prevent them replacing the latest stable release; npm continues using its beta dist-tag.
• The backend consumes this beta and removes its temporary refund-field type extensions. Backend draft PR: lifinance/lifi-backend#8948.

Verification

• pnpm exec eslint src: passed with four existing warnings.
• pnpm typecheck: passed.
• pnpm build: CJS, ESM and declarations passed.
• Throwaway consumer compiled against generated declarations: all seven request/action surfaces accept a string recipient or omission and reject a numeric recipient. Smoke file removed.
• No on-chain transactions submitted.

Checklist before requesting a review

• I have performed a self-review and testing of my code.
• This pull request is focused and addresses a single problem.
• Public-docs documentation updated. Backend OpenAPI and public type comments are updated; public-docs has not been changed.

Release note

Beta only; no stable npm release or production backend deployment is requested.

Publication verified: Release & Publish run succeeded; npm beta is 18.6.0-beta.0, latest remains 18.5.0, and the GitHub release is marked as a prerelease. Backend consumption passed root typechecking, pre-push checks and 27 targeted API/refund tests.