Fix flaky conflicts_with brew spec

[MikeMcQuaid]

Sorbet's runtime validate_call uses bind_call to invoke the original formulae_by_full_name method, bypassing RSpec mocks. Replace the mock with a real formula that declares conflicts_with "mysql55" via stub_formula_loader.

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[Copilot]

Pull request overview

Fixes a flaky conflicts_with spec by avoiding an RSpec mock that can be bypassed by Sorbet runtime validate_call, and instead using a real stubbed formula definition that exposes conflicts_with through the normal code path.

Changes:

• Replace formulae_by_full_name mocking with stub_formula_loader + a stub formula declaring conflicts_with "mysql55".
• Keep the rest of the install/upgrade/service expectations unchanged while making the spec exercise the real formula hashing path.

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