chore: expose structured stx fetch error data

[micaelae]

Explanation

Problem

The sentinel API returns structured error data for some fetch errors but they get stringified. This requires consumers to parse details from the Error message

Solution

Add the raw JSON data to the Error object so it can be accessed through error.data

References

Fixes https://consensyssoftware.atlassian.net/browse/SWAPS-5090

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
Small change to error shape for failed HTTP fetches; consumers that parsed the old message string need to use error.data instead.

Overview
When Sentinel API calls fail with a non-OK HTTP response, handleFetch no longer embeds the full JSON body in the thrown Error message. It now throws Fetch error: {status} and attaches the response body (keys camelCased via mapKeysToCamel) on error.data, so callers can read structured fields without parsing the message string.

Unit tests cover success, network failure, and non-OK responses (including the existing console.log of the response). The package changelog notes this under Unreleased.

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