Send bundle check failures to stderr

[MikeMcQuaid]

• Keeps brew bundle env --check output usable for shell capture.
• Treats unmet dependency reports as diagnostics instead of data.
• Covers the failure stream contract in check_subcommand_spec.

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[Copilot]

Pull request overview

Routes brew bundle check failure diagnostics to stderr so stdout remains suitable for scripting/capture (e.g., brew bundle env --check), and updates the spec suite to enforce the new stream expectations.

Changes:

• Send unmet-dependency and guidance messages from brew bundle check to stderr instead of stdout.
• Update check_subcommand_spec expectations from to_stdout to to_stderr, including a contract check for stdout behavior in at least one failure case.

Reviewed changes

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

File	Description
Library/Homebrew/bundle/subcommand/check.rb	Writes dependency-failure messaging to stderr and preserves stdout for success output.
Library/Homebrew/test/cmd/bundle/check_subcommand_spec.rb	Adjusts specs to assert failure output is emitted on stderr (and refines output stream contract coverage).

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