Add error response deserialization resilience

[davidkallesen]

Summary

• Fall back to raw string when error response deserialization fails
• Prevent RestClientDeserializationException on malformed 4xx/5xx bodies
• Success (2xx) deserialization failures still throw as before

Changes

🐛 Fixes

• Swallow deserialization errors for error responses (4xx/5xx)
• Return raw string content as ContentObject on failure
• Preserve throwing behavior for success (2xx) responses

♻️ Refactoring

• Inline EndpointResponse construction into return statements

✅ Tests

• Update test to verify fallback behavior instead of exception
• Rename test to reflect new expected behavior

📝 Documentation

• Document error response deserialization resilience in README
• Add code example showing ProblemDetails fallback scenario

🔧 Configuration

• Add .claude/settings.local.json to .gitignore