chore: Extract schema validation test helper to reduce boilerplate

[jflowers]

Summary

Extract the repeated schema compilation and validation boilerplate into a reusable test helper function.

Related: #118, #140, #141, #142

Problem

The schema compilation and validation pattern is repeated 7 times across internal/report/report_test.go (lines 215-241, 354-378, 416-441, 448-460, 465-527, 533-597, 604-667). Each instance has the same boilerplate:

sch, err := jsonschema.UnmarshalJSON(strings.NewReader(schemaJSON))
// error check
compiler := jsonschema.NewCompiler()
err = compiler.AddResource("schema.json", sch)
// error check
compiled, err := compiler.Compile("schema.json")
// error check
err = compiled.Validate(document)
// error check

With #140 (QualitySchema exposure), #141 (CRAPSchema), and #142 (round-trip tests), this count will grow to 12-15+ instances.

Proposed Change

Extract a test helper:

func compileSchema(t *testing.T, schemaJSON string) *jsonschema.Schema {
    t.Helper()
    sch, err := jsonschema.UnmarshalJSON(strings.NewReader(schemaJSON))
    if err != nil {
        t.Fatalf("failed to parse schema JSON: %v", err)
    }
    compiler := jsonschema.NewCompiler()
    if err := compiler.AddResource("schema.json", sch); err != nil {
        t.Fatalf("failed to add schema resource: %v", err)
    }
    compiled, err := compiler.Compile("schema.json")
    if err != nil {
        t.Fatalf("failed to compile schema: %v", err)
    }
    return compiled
}

Uses standard library testing package only — no third-party assertion libraries.

Scope

• Test-only refactor — no production code changes
• Update existing 7 call sites to use the helper
• All new schema tests (feat: Create CRAPSchema constant and expose via gaze schema crap #141, test: Add round-trip schema validation tests for QualitySchema #142) should use the helper from the start
• Low priority — mechanical refactor with no behavioral change