Improve Tab.for_name to use keg where possible.

[MikeMcQuaid]

I noticed a tiny performance improvement here when reviewing #21351 where we can avoid creating a Formula when loading a Tab from a name/rack path.

[Copilot]

Pull request overview

This PR optimizes Tab.for_name to avoid unnecessarily creating a Formula object when loading a Tab from a name/rack path, by preferring to use a Keg directly when one is available.

• Extracts common keg selection logic from Formulary.from_rack into a new Keg.from_rack method
• Modifies Tab.for_name to call Keg.from_rack first and only fall back to Formulary.from_rack when no keg exists
• Updates Formulary.from_rack to accept an optional keg parameter to avoid redundant keg lookups

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Library/Homebrew/keg.rb	Adds new Keg.from_rack class method that returns the linked, optlinked, or latest keg from a rack
Library/Homebrew/tab.rb	Refactors Tab.for_name to use Keg.from_rack first, avoiding formula creation when a keg is found
Library/Homebrew/formulary.rb	Updates from_rack to accept optional keg parameter with default value, eliminating duplicate keg selection logic

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[Rylan12]

Nice, makes sense!