Add and preserve the marketplace auto-update setting (#61286)

## What changed

- Add `marketplaces.<name>.auto_update` to the configuration type and schema, defaulting to `false`.
- Preserve `auto_update` when re-adding a marketplace with the same source type and source, including equivalent normalized local paths. Drop it when the source changes.
- Preserve existing marketplace entries when converting an inline `marketplaces` table for editing.

## Testing

Add regression tests for re-adding the same Git source, replacing it with another source, and re-adding an equivalent local path.

GitOrigin-RevId: 8a4108f1c8fc5a20a8564ead5baeef57e7ccfa19

codex-rs/core-plugins/src/marketplace_upgrade.rs

@@ -225,12 +225,11 @@ fn parse_configured_git_marketplace(
         .try_into::<MarketplaceConfig>()
         .map_err(|err| format!("invalid configured Git marketplace: {err}"))?;
     let MarketplaceConfig {
-        last_updated: _,
-        last_revision: _,
         source_type,
         source,
         ref_name,
         sparse_paths,
+        ..
     } = marketplace;
     if source_type != Some(MarketplaceSourceType::Git) {
         return Ok(None);