fix(plugin): detect updates for plugins without persisted install sources

[xiaoyuyu6420]

Fixes #9668

Problem

For plugins installed before the install-source persistence feature (PR #9037, v4.26.2), there is no persisted plugin_install_sources record. The backend falls back to an implicit source record (implicit: true) as a display-only placeholder.

The frontend update check (checkUpdate) skipped any extension whose install_source.implicit === true, so these legacy plugins never surfaced an available update — the user had to manually reinstall or re-bind the plugin source before the "update available" badge appeared.

Root cause

• checkUpdate bails out early on source.implicit === true.
• findMarketPluginForExtension also returns null for implicit sources, so even the update download URL could not be resolved.

Fix

Include implicit sources in the update check instead of skipping them:

• Implicit records already carry install_method: "market" with a null registry_url, which resolves to the default registry. The existing matching logic then looks the plugin up in the default market by market_plugin_id → repo → name. Only a real match surfaces an update, so no false positives are introduced for unmatched plugins.
• The update action still routes implicit plugins through the existing "bind source" dialog (via buildUpdateContext), so updating remains safe and the persisted record is created on confirm — matching the previously working "reinstall/change source" path.

Also fixes the misspelled online_vesion → online_version field in serialize_plugin_base so the backend payload matches what the frontend reads.

Testing

• node --check dashboard/src/views/extension/useExtensionPage.js passes.
• Backend change is a field rename only; no test references the old name.

Summary by Sourcery

Restore update detection for legacy market plugins while correcting the plugin version payload field.

Bug Fixes:

• Detect available updates for legacy plugins without persisted install-source records.
• Correct the serialized plugin version field name so frontend update data is populated correctly.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Approved.

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