Fix dynamic completion audit crash

[MikeMcQuaid]

Fixes #22379

• Avoid crashing on formulae that build shells: from a loop.
• Keep the cop limited to literal shell lists it can safely rewrite.
• Cover the rustup completion pattern that exposed the issue.

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[Copilot]

Pull request overview

This PR fixes a crash in the FormulaAudit/SingleGenerateCompletionsDSLCall RuboCop cop when it encounters generate_completions_from_executable calls whose shells: argument isn’t a simple literal list (e.g., shells: [shell] from a loop), as reported in #22379.

Changes:

• Prevent the cop from crashing by only collecting shells when it can safely extract a literal shells: [:bash|:zsh|:fish] value.
• Add an RSpec case covering the dynamic-shell loop pattern (similar to the rustup completion pattern that triggered the crash).

Reviewed changes

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

File	Description
Library/Homebrew/rubocops/lines.rb	Makes shell extraction nil-safe so non-literal shells: usage doesn’t raise during auditing.
Library/Homebrew/test/rubocops/lines/generate_completions_spec.rb	Adds coverage ensuring the cop reports no offense (and does not crash) for dynamically generated shells:.

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