fix: include quote slippage value in MixPanel event properties

[micaelae]

Explanation

Saves the QuoteResponse["quote"]["slippage"] value in the txHistory to populate the Submitted, Failed and Completed MixPanel events

References

Fixes https://consensyssoftware.atlassian.net/browse/SWAPS-4446

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
Single-field analytics/history fix with a safe fallback; no submission or transaction logic changes.

Overview
Bridge tx history now records the quote’s slippage instead of always storing 0, so Unified SwapBridge Submitted, Failed, and Completed MixPanel events can report the correct slippage_limit (via persisted slippagePercentage).

On SubmitStep.AddHistoryItem, slippagePercentage is set from quoteResponse.quote.slippage, with ?? 0 when the field is missing. Tests and changelog were updated to match.

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