cask: merge multiple conflicts_with stanzas

[hyuraku]

conflicts_with can only appear once per cask: a second top-level stanza raises, and one in an on_* block replaces the top-level one. So a cask can't conflict with one cask everywhere and another on one
OS only. Cask::DSL has a TODO asking to merge instead, and the Cask Cookbook already documents multiple occurrences as allowed.

Now each call merges into the previous ones as a set union, and the NoOverrides cop no longer flags conflicts_with.

No behavior change for existing casks: none of the 7,691 casks in homebrew-cask uses it more than once, and merging is idempotent so reloading gives the same result. Added a fixture cask, a DSL spec, and a NoOverrides cop spec.

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AI (Claude Code) assisted in writing the code and tests.
I reviewed the change and ran brew lgtm locally

[MikeMcQuaid]

Thanks again @hyuraku!