Compact install step JSON (25/26)

[MikeMcQuaid]

Needs #23199

Reduce the internal JSON representation of structured install steps.

• omit values already supplied by runner defaults
• preserve explicit compatibility-sensitive behaviour
• cover compact defaults in the install-step specs

AI disclosure: using OpenAI Codex 5.6 Sol max with local review and testing.

[Copilot]

Pull request overview

This PR reduces the size of the internal JSON representation for structured install steps by omitting fields that are implied by runtime defaults, while keeping explicit values where they change behaviour. It also updates and adds specs to ensure the compact representation is produced.

Changes:

• Omit default-valued fields in the install-step DSL output (e.g., overwrite, match, attempts, group) while preserving non-default/behaviour-changing values.
• Update the install-step runner to handle omitted match for terminate_process.
• Extend tests to assert omission of default keys and update existing JSON expectations.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Library/Homebrew/install_steps.rb	Adjusts step serialization to omit runtime-default fields and updates runtime handling for omitted match.
Library/Homebrew/test/install_steps_spec.rb	Adds coverage asserting that default runtime keys are omitted from generated steps.
Library/Homebrew/test/formula_spec.rb	Updates expected JSON output to reflect omission of default overwrite: false for symlink steps.

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