Search: Fix empty folder details for nested folder items

[papagian]

What is this feature?
For nested folders we have to refer to the folder table for getting the parent folder details instead of the dashboard table
that knows only of flat folders. However, in the search query we used to join with the latter on folder_id column. As a result parent folder details were missing for a subfolder.

The sequential IDs for a folder differ in folder and dashboard table: but their UIDs are the same.

This fix:

• Introduces a schema migration for adding a folder_uid column in dashboard table
• Introduces a data migration for setting folder_uid into the existing dashboard and folder rows in the dashboard table. (this can take several minutes for large setups > 1M dashboards)
• Stores folder_uid when saving a dashboard or folder
• Modifies the search query to refer to the folder table if the nested folders are enabled
• Adds tests for searching by folder and title with nested folders on/off

Why do we need this feature?

This is introduced for fixing search results for the nested folders feature but we need it in general because in the future we want to exclusively use the folder table for storing folders and without this migration this is not possible.

Who is this feature for?

[Add information on what kind of user the feature is for.]

Which issue(s) does this PR fix?:

Fixes #74433

Special notes for your reviewer:
To reproduce the issue:

• enable nested folders in main
• create a folder hierarchy like:

• the folder details are missing from api/search?query=New response
• the folder details are missing from api/search?query=child response
• switch to this branch and try the above request: make sure that the folder details are not missing

Try also moving a subfolder under another folder and to the root and make sure that search response is correct.

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[papagian]

/deploy-to-hg

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[papagian]

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[papagian]

/deploy-to-hg

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[papagian]

/deploy-to-hg

[ephemeral-instances-bot]

• Preparing your instance. A comment containing your instance's url will be added to this PR when the instance is ready.
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