fix: merge account balances preserve fields

[khanti42]

Explanation

Fix merge-mode balance updates to shallow-merge per asset instead of replacing the whole balance object.
Amount-only updates no longer drop existing fields (e.g. enrichment metadata).

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