[fy-god]

Closes #8

What changed

• Raise IrisRequestError for non-OK fee responses and preserve the HTTP status.
• Reject successful fee responses that contain no fast-transfer tier at or below the 1000 finality threshold, including the source and destination domains in the error.
• Keep the existing fee result for normal responses, including the exact 1000 threshold boundary.
• Extend mocked-fetch coverage for the HTTP status and missing-tier cases.

Verification

• pnpm test — 9 files, 113 tests passed
• pnpm run typecheck
• pnpm run build

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