feat(perps): add PREVIOUS_LEVERAGE to Perps analytics event properties

[abretonc7s]

Explanation

Perp UI Interaction leverage_changed events need a Segment property previous_leverage. Clients import keys from PERPS_EVENT_PROPERTY in @metamask/perps-controller; that object had LEVERAGE, LEVERAGE_USED, and DEFAULT_LEVERAGE but not PREVIOUS_LEVERAGE. Mobile TAT-3743 currently ships an interim local constant so payloads are not unknown camelCase previousLeverage.

This change adds PERPS_EVENT_PROPERTY.PREVIOUS_LEVERAGE (previous_leverage) next to the other leverage keys, documents it on Perp UI Interaction leverage_changed in the MetaMetrics reference (eventNames.ts), and records it in the Unreleased changelog so the next published @metamask/perps-controller version includes the key. PERPS_EVENT_PROPERTY is already a package-root export; no new export path is required. This is additive and non-breaking.

Validation: targeted Jest on eventNames.test.ts (63 passing), changelog validate, changed-file ESLint/oxfmt, and a headless recipe that imports the constant and asserts strict equality.

References

• https://consensyssoftware.atlassian.net/browse/TAT-3767
• Relates to TAT-3743 (Mobile interim PERPS_ANALYTICS_PREVIOUS_LEVERAGE). After this package is published, Mobile should bump @metamask/perps-controller and drop that interim constant.
• No Extension follow-up: Extension does not emit this property.

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 — not applicable: additive as const key, same pattern as PERPS_MODE / ROE_SIGN

Screenshots/Recordings

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Note

Low Risk
Additive analytics constant and documentation only; no runtime trading, auth, or persistence behavior changes.

Overview
Adds PERPS_EVENT_PROPERTY.PREVIOUS_LEVERAGE (previous_leverage) so Perp UI Interaction leverage_changed events can send the prior leverage using the shared @metamask/perps-controller contract instead of client-local constants.

Documents the property on LEVERAGE_CHANGED in eventNames.ts, records it under Unreleased in the package changelog, and adds a Jest assertion that the key exports as previous_leverage.

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