Implement `RateLimitError`

[codebestia]

Description

Raised on HTTP 429 responses when retries are exhausted or disabled. Exposes retry_after (seconds) so the caller can implement their own wait logic.

Proposed Steps

• Define RateLimitError(ShadeError) in errors.py.
• Add retry_after: Optional[int] attribute.
• The HTTP client populates this from the Retry-After header before raising.

Acceptance Criteria

• A 429 response raises RateLimitError.
• error.retry_after reflects the Retry-After header value.
• When Retry-After is absent, error.retry_after is None.

[Rachaelisa]

Hello, I’ve gone through the requirements and I’m confident I can implement a proper solution that matches the acceptance criteria. Kindly assign this issue to me.