fix: bubble up error from failed sub-repository size calculation

What does this MR do?

After investigating #1360 (closed) we identified the root cause of the sentry error only the size of root repositories can be estimated :

• The error occurs when size computations for sub-repositories time out with a SQL statement timeout error
• As a fallback, the system attempts to estimate the size, but size estimation is only supported for root repositories and so it reports only the size of root repositories can be estimated to Sentry

Fix:

• Update the code to only call EstimatedSizeWithDescendants only for root repositories and not for sub-repositories
• Specifically, modify this code to check if repo.IsTopLevel() before proceeding with the size estimation
• Let the error bubble up unchanged from there

This makes sure the original SQL timeout error (ERROR: canceling statement due to statement timeout) will be the one that bubbles up and gets reported and not the only the size of root repositories can be estimated which is expected behaviour.

Related to #1360 (closed)

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Closes #1360 (closed)