Preserve structured app validation issues internally

[dmerand]

What

Preserve structured app validation issues internally during app loading.

This keeps the current CLI behavior the same while retaining structured issue metadata alongside the rendered validation messages.

Why

The new app validate --json mode is a useful start, but the validation pipeline still flattens schema issues into strings too early.

To support repair loops and future automation work, the CLI needs to retain structured validation details before deciding how to render them publicly.

How

• add structured issue parsing alongside the existing human-readable formatter
• preserve structured issue metadata at the loader/report boundary
• update AppErrors to retain both rendered messages and structured issues
• keep current text output and existing public behavior unchanged

Testing

To test manually:

1. Run shopify app validate against invalid app or extension config and confirm the visible CLI output is unchanged.
2. Review the focused loader and parsing test coverage for schema errors, union errors, and repeated error writes for the same file path.

[dmerand]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Expose structured app validation issues in JSON output #7070
• Preserve structured app validation issues internally #7066 👈 (View in Graphite)
• Add --json to validate #7065
• --json -j should not use color #7069
• main

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[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟢	Statements	82.22%	14967/18204
🟡	Branches	74.69%	7417/9931
🟢	Functions	81.29%	3781/4651
🟢	Lines	82.61%	14149/17128

Test suite run success

3924 tests passing in 1506 suites.

Report generated by 🧪jest coverage report action from 3c313f8