fix: parse compact release metadata in installer

[efrazer-oai]

Summary

GitHub's latest-release endpoint can return compact, single-line JSON. The standalone installer treated release metadata as line-oriented text, so those responses could make asset lookup fail even though the requested assets were present.

The regression was introduced by #31056. That change reused the /releases/latest metadata response for both version resolution and asset lookup, exposing the existing formatting-sensitive asset parser to compact responses from that endpoint.

This change parses the release metadata once with a one-pass POSIX awk scanner. The scanner tracks JSON strings and nesting, extracts the root release tag plus direct asset name/digest pairs, and produces the same result regardless of whitespace or object field order. It uses POSIX fold to bound awk record sizes so compact responses stay fast across awk implementations.

Fixes #31520.

Changes

• replace line-oriented release metadata matching with structure-aware parsing
• reuse the parsed metadata for latest-version and asset-digest lookup
• add regression coverage for compact JSON, reordered fields, nested decoys, and JSON-looking release text

Design decisions

• Keep the installer dependency-free by using standard POSIX tools already required by the shell installer.
• Parse only the GitHub release fields the installer consumes, in one pass, instead of vendoring a general JSON library.
• Preserve asset-object boundaries so nested or string-encoded name and digest fields cannot be mistaken for release assets.

Testing

• Tests: focused installer suite locally and on Linux.
• Smoke tests: real pretty and compact GitHub release metadata, latest-release resolution, and checksum-asset selection.
• Portability: macOS awk plus Linux gawk, mawk, and nawk.
• Stress coverage: randomized formatting and field order, adversarial nested/string content, and a synthetic 2,000-asset compact response.

[etraut-openai]

Looks good! I had codex also run some manual tests on this, and they all passed.

Thanks for doing this.