chore: add unknown to feature ids

[infiniteflower]

Explanation

FeatureId in @metamask/bridge-controller enumerates the client experiences that request quotes and emit Unified Swap/Bridge analytics (e.g. perps, unified_swap_bridge, dapp_swap). There was no explicit value for cases where the originating feature cannot be determined, so callers had to reuse an existing ID (typically UNIFIED_SWAP_BRIDGE) or omit featureId entirely and rely on controller fallbacks.

This PR adds FeatureId.UNKNOWN = 'unknown' to the enum. The string value follows the existing snake_case segment convention used by other FeatureId members. Callers can now pass FeatureId.UNKNOWN explicitly to fetchQuotes, quote requests, and analytics events when the feature context is unavailable.

No controller behavior changes are included. Sorting, quote-request overrides, and metrics fallbacks (which still default to UNIFIED_SWAP_BRIDGE when featureId is absent) are unchanged. This is an additive, non-breaking export.

References

• Related to feat: add feature_id and batch_id to Unified SwapBridge events #8964 (introduced FeatureId and required feature_id on analytics events)

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
Enum-only additive export with no runtime or controller logic changes; low risk aside from consumers optionally adopting the new analytics value.

Overview
Adds FeatureId.UNKNOWN ('unknown') to @metamask/bridge-controller so clients can tag quote fetches and Unified Swap/Bridge analytics when the originating UI/feature cannot be identified, instead of reusing another FeatureId or omitting the field.

This is an additive, non-breaking type export only; controller defaults and metrics behavior when featureId is missing are unchanged. The Unreleased changelog entry documents the new enum member.

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