brew services: use package-provided service file when no run command

[Yuxin-Qiao]

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What this does

brew services start/restart again uses the service file installed by the formula (the package-provided plist/systemd unit) when the formula's service do block does not define a run command.

PR #23312 changed install_service_file to always regenerate the service file from the service do block via service_contents. Formulas that ship their own service file only define name (without run), so the regenerated plist had an empty ProgramArguments and launchctl bootstrap failed with exit code 5 (Input/output error). Examples in homebrew-core: dbus, xinit.

This restores the documented contract from the Formula Cookbook: if name is defined without run, Homebrew makes no attempt to change the package-provided service file. It matches the logic already used by FormulaInstaller#install_service, which only writes a generated plist when formula.service.command? is true.

The per-service environment variable overrides from #23312 still apply whenever Homebrew generates the service file (i.e. when a run command is defined).

Why

PR #23312 unintentionally dropped support for package-provided service files, breaking brew services start for formulae that ship their own plist/systemd unit (e.g. dbus, xinit in homebrew-core). The generated file is invalid for such formulae because the service do block contains only name, so launchctl bootstrap rejects it with exit code 5.

Step-by-step reproduction

brew install zhyu/tap/roused
brew services start roused

Before this fix:

Bootstrap failed: 5: Input/output error
Error: Failure while executing; `/bin/launchctl bootstrap gui/501 /Users/.../io.github.zhyu.roused.plist` exited with 5.

How it was verified

• brew tests --only=services --no-parallel: 119 examples, 0 failures
• brew style on the changed files: no offenses
• brew typecheck: no errors
• New unit test covers the fallback to the installed service file

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

AI (Claude Code via Codex) was used to analyze the issue, implement the fix, and prepare this PR. The change was reviewed, typechecked with Sorbet, linted with RuboCop, and verified with the services test suite locally before submission.

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Fixes #23408

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