Handle shell startup output in installed package JSON

[joseph1020]

Summary

Allow the Installed repository to decode brew info --installed --json=v2
when interactive-shell startup output appears before Homebrew's JSON.

This was reproduced with iTerm2 shell integration emitting OSC 1337 sequences
before an otherwise valid Homebrew JSON payload.

Changes

• Preserve the existing login/interactive shell behavior.
• Recover the Homebrew JSON payload after leading shell startup output.
• Add a regression test for JSON-like shell output before the real payload.

Testing

• Syntax-checked the changed Swift files.
• git diff --check.
• Targeted test not run locally due to local toolchain limitations.

PR checklist

• Have you followed this repository's contribution and workflow guidance?
• Have you explained what changed and why this should land now?
• Have you run relevant local checks for the changed scope?
• Are changes scoped and free of unrelated modifications?

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• AI was used to generate or assist with generating this PR.
• ChatGPT and OpenAI Codex assisted with reproducing the failure, reviewing
  the decoding boundary, and drafting the regression and implementation.
  I manually reproduced the issue and reviewed the final diff.

[MikeMcQuaid]

We will not be merging any community PRs until after Homebrew 7.0.0 is released.