fix(assets-controller): re-run snap discovery on snap install

[salimtb]

Subscribe to SnapController:snapInstalled so that keyring snap chain discovery is re-triggered whenever a new snap is installed. This ensures snap-backed chains (Bitcoin, Solana, Tron, etc.) appear in activeChains immediately after install without waiting for a permission state change.

Explanation

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
Adds a new required messenger event (SnapController:snapInstalled) for SnapDataSource, which is a breaking integration change for hosts using restricted messengers; functional logic change is small but could prevent snap chain discovery if not wired.

Overview
Ensures snap-backed chains become available immediately after installing a new snap by having SnapDataSource subscribe to SnapController:snapInstalled and re-run keyring snap discovery.

Updates messenger typing so SnapControllerSnapInstalledEvent is included in SnapDataSourceAllowedEvents, and documents the resulting breaking requirement in the assets-controller changelog for hosts that restrict allowed events.

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