fix(client/oauth): preserve token endpoint error diagnostics

[junyuanz1]

Token endpoint failures currently discard the OAuth error description, leaving callers with only an HTTP status. For example, Notion's invalid_request response explaining that a client used multiple authentication methods becomes only Token endpoint returned status 400., forcing callers to replay the request to diagnose it.

Preserve RFC 6749 §5.2 error and error_description in the exception message and expose http_status, error, and error_description readers. Keep InvalidGrantError classification and refresh recovery behavior unchanged. Malformed responses retain the status-only fallback.

Only the two diagnostic fields are included; other response fields and the raw body are excluded. Normalize text to the RFC's printable ASCII set and cap error codes at 128 characters and descriptions at 512. Document that provider-controlled descriptions may still contain sensitive information and require application logging policies.

Validation:

• Full test suite: 1,789 tests, 4,764 assertions, no failures or errors (Ruby 3.4.5).
• Regression coverage for authorization-code and refresh failures, invalid grants, malformed bodies, unexpected field types, sanitization, truncation, and exclusion of unrelated response fields.
• git diff --check passed.
• RuboCop reports 30 existing offenses; no new offenses introduced.

[koic]

@junyuanz1 ping.

[junyuanz1]

@koic updated :) sorry missed the email notificatoin on Friday

[koic]

@junyuanz1 Looks good to me. Can you squash your commits into one?

[junyuanz1]

@koic done :)

[koic]

Thanks!