fix(core): improve API response error handling and retry logic

[mattKorwel]

• Always retry on transient network error codes (e.g., ETIMEDOUT, ECONNRESET) regardless of retryFetchErrors setting.
• Update WebFetchTool to use retryWithBackoff for fallback fetch operations to handle rate limits (429) and server errors (500s) robustly.
• Add comprehensive tests for retry logic and WebFetchTool error handling.