Fixed trimming of favorite namespaces in Config

[assert-0]

Summary

Fixed the way favorite namespaces are trimmed when validating and merging cluster configs.
Related issue: #2978

Changed

• Namespace.merge now trims the favorite namespaces list after merging with the old Namespace object
• fixed trimming logic so it doesn't delete elements of the list it is iterating over (anti-pattern)
• extracted trimming logic into private helper method Namespace.trimFavNs

Added

• Additional test case in internal/config/data/context_int_test.go for merging two oversized Namespace objects