chore: relax RelatedAsset validation and filter invalid assets

[zone-live]

Explanation

Fixes a production bug where one relatedAsset missing sourceAssetId caused the entire What's Happening section to show an error state.

• Make name and sourceAssetId optional in RelatedAssetStruct
  and RelatedAsset type (symbol is now the only required field)
• Replace normalizeRelatedAssets with filterAndNormalizeRelatedAssets
  which strips assets with empty symbol and drops trends with zero
  valid assets after filtering

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

Medium Risk
Changes public RelatedAsset typing and silently drops trends/assets; consumers must handle optional name/sourceAssetId and possibly fewer trends than the raw API sent.

Overview
Fixes a production failure where fetchMarketOverview rejected the whole market-overview payload when any relatedAsset lacked name or sourceAssetId, breaking the What's Happening UI.

RelatedAsset validation and types now treat name and sourceAssetId as optional; symbol is the only required field. Post-validation normalization was renamed to filterAndNormalizeRelatedAssets: it still fills missing caip19 with [], removes assets with empty symbol, and drops trends that end up with no valid related assets (overview can return trends: [] instead of throwing). Changelog and service tests cover the new behavior.

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