feat: adds asset picker opened metric event

[ghgoodreau]

Explanation

Adds an AssetPickerOpened metric event.

References

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
Metrics-only/type updates with minimal runtime behavior change, limited to event-name plumbing and property composition.

Overview
Adds a new UnifiedSwapBridgeEventName.AssetPickerOpened metrics event and types its required payload to include asset_location: 'source' | 'destination'.

Updates BridgeController.#getEventProperties to treat this event as controller-state-independent (returns only base/client properties) and documents the addition in the bridge-controller changelog.

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