fix(vega-backend): allow catalog connector config updates

[kenlee1988]

Pull Request

What Changed

Brief description of changes:

• Remove the connector_config immutable-field check from catalog update.
• Allow updating catalog connector settings such as database, host, and related connection fields while keeping connector_type and enabled protected.
• Add a handler test proving connector_config.database can be changed through PUT /catalogs/{id}.
• Update the Catalog API docs to describe deferred stale marking on the next discover alignment.

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Why

• Related Issue: 【vega-backend】【catalog】编辑catalog时，修改database请求报错 #393
• Background: Updating a catalog's database previously failed because connector_config.database was treated as immutable. The expected behavior is that catalog config updates should succeed, and old resources should be reconciled by the next discover task.

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How

• Key implementation points:
  • Removed the immutableFields validation block from updateCatalog.
  • Kept the existing connector_type immutability guard.
  • Kept the existing enabled guard, so enable/disable still uses dedicated endpoints.
  • Documented that stale resource alignment happens later during discover instead of during catalog update.
• Breaking changes (API, config, database, etc.):
  • Previously rejected connector config changes are now accepted when connector_type and enabled remain unchanged.

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Testing

• Unit tests passed locally
• Integration tests passed locally
• Verified in test environment

Test notes:

go test ./driveradapters ./logics/catalog ./logics/resource

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Risk & Rollback

• Potential risks: Clients can now update connector settings that used to be rejected, so follow-up discover runs are responsible for reconciling resources.
• Rollback plan: Restore the connector config immutable-field validation block and revert the related test/doc updates.

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Additional Notes

• PR diff currently affects catalog_handler.go, catalog_handler_test.go, and catalog.yaml.