fix(validator): report not schema resolution errors

[risu729]

Summary

• add regression coverage for an invalid $ref used directly as a not subschema
• surface direct not subschema resolution errors as lint diagnostics
• resolve the schema item without holding its write lock across validation
• use assertion semantics for warning-only subschema results
• verify normal inline not match and non-match behavior

Bug case

Given this schema:

{
  "type": "object",
  "properties": {
    "value": {
      "type": "string",
      "not": {
        "$ref": "#/$defs/missing"
      }
    }
  }
}

and this TOML document:

value = "foo"

the $ref cannot resolve because #/$defs/missing does not exist. Resolving the schema item assigned to not returns Err(InvalidJsonPointer).

Previously, validate_not used if let Ok(Some(...)) around resolution. The error was only logged, did not match the Ok(Some(...)) pattern, and execution fell through to a successful validation result. As a result, every value passed lint even though this not subschema could not be evaluated. This was a fail-open reference-resolution bug, not a general problem with inline not constraints.

The new implementation matches the direct not schema-item resolution result explicitly. A resolution Err becomes a user-facing diagnostic with the original error code and message, so this case now fails lint with an invalid-json-pointer error. Resolved schemas continue through normal assertion evaluation, including treating warning-only results as assertion success.

Ok(None) keeps the existing unresolved/offline behavior.

Scope

This change covers errors returned while resolving the schema item directly assigned to not. It does not reclassify resolution errors produced later while evaluating deeper descendant or composite schemas; that requires validator-wide separation between assertion failures and schema-evaluation failures.

Validation

• cargo fmt --all -- --check
• cargo check -p tombi-validator
• cargo test -p tombi-validator (20 passed)
• cargo test -p tombi-linter --test integration not_schema:: (4 passed)
• cargo test -p tombi-linter --test integration (284 passed)
• cargo clippy -p tombi-linter --test integration -- -D warnings

The regression test and implementation fix are kept in separate commits. Follow-up review fixes add diagnostic metadata assertions and coverage for resolved inline not behavior.

[Copilot]

Pull request overview

not サブスキーマ内の $ref 解決に失敗した場合に、従来は fail-open で lint が成功してしまっていた挙動を修正し、解決エラーをユーザー向け診断として返すようにする PR です。TOML のスキーマ検証/リンタの正確性（特に参照解決失敗時の扱い）を改善します。

Changes:

• not サブスキーマ解決結果を明示的に match し、解決 Err を診断として返す（fail-closed化）
• 未解決 Ok(None) の既存挙動を維持しつつ、解決成功時は従来通り not の評価を継続
• 未解決 $ref を not に含む回帰テスト用スキーマ/統合テストを追加

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
schemas/not-schema-test.schema.json	not 内の未解決 $ref を再現するためのテスト用 JSON Schema を追加
crates/tombi-validator/src/validate/not_schema.rs	not のサブスキーマ解決エラーを lint 診断として返すように修正
crates/tombi-linter/tests/integration/not_schema.rs	未解決 $ref が診断として報告されることの回帰テストを追加
crates/tombi-linter/tests/integration.rs	新規統合テストモジュール not_schema をテストスイートに追加

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[ya7010]

Thank you.

In cases where the writing style cannot be resolved by JSON Schema itself, I have been treating it as a warning until now, but perhaps we should change it to a strict error overall.

[risu729]

Thanks for the quick response! I'm not too sure about that tbh, but for this specific case, the warning was a problem, I think.