install: isolate formula and cask fetch errors

[MikeMcQuaid]

• One malformed formula or cask (e.g. a bottle root_url that is an invalid URI) raised while enqueueing downloads and aborted the whole brew upgrade batch with an internal stack trace.
• Rescue anything raised by an individual formula's or cask's prelude and enqueue steps so the offending package is reported and skipped, the rest still proceed and the command exits nonzero.

Fixes #23377.

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Claude Fable 5 with local review and testing.

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[Copilot]

Pull request overview

This PR improves the resilience of brew install/brew upgrade batch fetch/enqueue flows by isolating per-formula and per-cask failures during download queue preparation, so one malformed package reports an error and is skipped while the rest continue (exiting nonzero overall).

Changes:

• Refactors formula fetch prelude/enqueue steps into a shared select_formula_installers helper that rescues per-installer failures and filters out offenders.
• Updates cask installer enqueue logic to rescue and skip failures during pre-fetch collection and during enqueue, allowing the batch to proceed.
• Adds RSpec coverage for “skip the bad item, continue with the rest” behavior for both formulae and casks.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
Library/Homebrew/install.rb	Adds per-item error isolation for formula and cask fetch/enqueue steps, filtering out failing items and continuing the batch.
Library/Homebrew/test/install_spec.rb	Adds specs asserting batch continuation when an individual formula/cask fetch/enqueue step raises.

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