attestation: remove T.must

[cho-m]

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This updates ensure_executable! return to drop nil and uses the with_env return so that Sorbet can infer the type.

Also search all guesses for ensure_executable! prior to installing formula.

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Change is done as usually better to avoid T.must as it adds some overhead for all users (as an extra function call even when Sorbet is disabled for non-developers) and is often a sign that code needs refactoring

[Copilot]

Pull request overview

This PR improves type safety in the attestation verification code by eliminating T.must assertions. It updates ensure_executable! to return a non-nilable Pathname type, allowing Sorbet to infer types correctly through the with_env block return value. The implementation is also improved to search all potential executable paths before attempting formula installation.

Key changes:

• Updated ensure_executable! signature to return Pathname instead of T.nilable(Pathname)
• Refactored implementation to use find(&:exist?) to search all candidate paths before installing
• Removed T.must assertion in gh_executable by leveraging with_env return value

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Library/Homebrew/extend/kernel.rb	Updated ensure_executable! signature and implementation to return non-nilable Pathname and search all paths before installing; fixed comment grammar
Library/Homebrew/attestation.rb	Removed T.must assertion by using with_env return value directly

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[MikeMcQuaid]

Nice cleanup!