chore: getErrorData util for Sentinel API

[micaelae]

Explanation

Export getErrorData util which extracts data from Sentinel API errors

References

Fixes https://consensyssoftware.atlassian.net/browse/SWAPS-5091

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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Note

Low Risk
Additive public utility and tests only; no change to fetch or controller success paths.

Overview
Adds and exports a public getErrorData helper so consumers can turn Sentinel/smart-transaction failures into user-facing or analytics strings without reimplementing fetch error parsing.

When handleFetch rejects, it attaches camel-cased JSON on a thrown object’s data field (not a true Error instance). getErrorData returns data.error when it’s a string, otherwise JSON-stringifies data, and falls back to Error.message or String(error) for ordinary errors.

Coverage includes unit tests for those branches and a getFees integration test asserting a 400 Sentinel response still rejects with message: 'Fetch error: 400' and structured data.error. The package changelog documents the new export.

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