Fix theme check JSON output for offenses without a position - #9041

WHY are these changes introduced?

shopify theme check --json fails when Theme Check reports an offense that it cannot place in the file. Theme Check gives line and character -1 for such an offense, for example when an empty layout/theme.liquid misses {{ content_for_header }}. The JSON projection adds 1 to each position, so the result is 0. The output schema requires one-based positions greater than 0, so the command prints a ZodError ("Number must be greater than 0", path issues, <n>, startRow) instead of the issues.

WHAT is this pull request doing?

It clamps each projected position to at least 1. An offense without a known position now points at the start of the file. The schema and its one-based contract do not change. A regression test covers an offense with -1 positions.

Output before, for a theme with an empty layout/theme.liquid:

{"error":{"type":"bug","message":"[\n  {\n    \"code\": \"too_small\",\n    \"minimum\": 0,\n    ...\n    \"message\": \"Number must be greater than 0\",\n    \"path\": [\n      \"issues\",\n      0,\n      \"startRow\"\n    ]\n  },\n  ..."}}

Output after:

{"valid":false,"issues":[{"filePath":"<dir>/layout/theme.liquid","check":"RequiredLayoutThemeObject","severity":"error","startRow":1,"startColumn":1,"endRow":1,"endColumn":1,"message":"The required object '{{ content_for_header }}' is missing in layout/theme.liquid"},{"filePath":"<dir>/layout/theme.liquid","check":"RequiredLayoutThemeObject","severity":"error","startRow":1,"startColumn":1,"endRow":1,"endColumn":1,"message":"The required object '{{ content_for_layout }}' is missing in layout/theme.liquid"}],"errorCount":2,"warningCount":0,"infoCount":0}

How to manually test your changes?

1. Create a theme directory with an empty layout file: mkdir -p /tmp/empty-theme/layout && touch /tmp/empty-theme/layout/theme.liquid.
2. Run pnpm shopify theme check --path /tmp/empty-theme --json.
3. Check that the command prints two RequiredLayoutThemeObject issues at row 1, column 1, and no ZodError.

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add