Restore use of package-provided service files

[MikeMcQuaid]

• Since Add per-service user environment variable overrides #23312, brew services start always regenerated the service definition from the formula service block. Formulae that bundle their own service file only declare name there, so the generated plist had empty ProgramArguments and launchctl bootstrap failed with Input/output error.
• Read the installed service file instead unless the service block defines a command, the same gate FormulaInstaller#install_service uses when writing generated service files into the keg.

Fixes #23408
Closes #23413

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Claude Fable 5 Max with local review and testing.

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[Copilot]

Pull request overview

Restores brew services start/restart behaviour to use a formula’s package-provided plist/systemd unit when the formula’s service block does not define a run command, avoiding generation of invalid service definitions (e.g. empty ProgramArguments on macOS).

Changes:

• Update FormulaWrapper#service_contents to read the installed service file when there is no service block or when the service block has no command, otherwise generate plist/unit content from the service block.
• Add unit tests covering both generation paths (launchctl/systemd) and the package-provided fallback cases.

Reviewed changes

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

File	Description
Library/Homebrew/services/formula_wrapper.rb	Makes service_contents fall back to reading the package-provided service file when no run command is defined.
Library/Homebrew/test/services/formula_wrapper_spec.rb	Adds coverage for service_contents generation and fallback behaviour.

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[ogiarch]

Can someone please provide a timeline for the next release containing this fix? This bug wasted hours for me yesterday and today trying to troubleshoot why Homebrew wouldn't start the mongodb-community service properly.

I am not able to downgrade Homebrew and have it still function in the way I need, so this is a serious issue.

[MikeMcQuaid]

@ogiarch We'll likely tag a new release on Monday.