feat: detect retryable errors through error.cause chain

[ShutovKS]

Проблема

statusCode/errorText смотрели только на error, error.data, error.response. Обёрнутые fetch-ошибки (new TypeError("fetch failed", { cause: { statusCode: 429 } })) прятали реальный код/текст в error.cause, и fallback не срабатывал.

Решение

Обе функции теперь проходят по цепочке error.cause (ограничение глубины 5, безопасно к циклам). 429/503, лежащий на один-несколько уровней глубже, снова триггерит fallback. Заодно вынесен numericField/ownStatusCode — меньше дублирования.

Тесты

Добавлены кейсы: статус на cause, вложенная цепочка cause.cause, Error с текстовым cause, циклическая ссылка (терминируется). Юнит-тесты 40/40, typecheck чист.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR improves the model-fallback plugin’s retryability detection by following error.cause chains so wrapped fetch-style errors can still trigger fallback, and adds tests to cover the new behavior.

Changes:

• Refactors status extraction into numericField/ownStatusCode and makes statusCode() recurse through error.cause with a max depth.
• Extends errorText() to incorporate Error.cause text (depth-limited) when the input is an Error.
• Adds helper tests for wrapped errors, nested cause chains, and circular cause chains.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/plugin.ts	Adds depth-limited cause traversal for status-code detection and enhances error text aggregation for Error instances.
test/index.test.ts	Adds unit tests covering retryability via error.cause, nested causes, and cycle termination.

Comments suppressed due to low confidence (1)

src/plugin.ts:231

• errorText only walks the cause chain when the value is an Error. For plain objects it falls back to JSON.stringify(error), which drops text if cause is an Error (JSON.stringify(new Error("x")) -> "{}"). That can miss retryable text buried in error.cause for non-Error wrappers. Consider handling Record values by extracting name/message (when present) and recursively appending cause text with the same depth cap.

  try {
    return JSON.stringify(error)
  } catch {
    return String(error)
  }

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