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.

[grantfox-oss]

🦊 GrantFox — @Rachaelisa has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #19)
2. Your PR will be reviewed by the ShadeProtocol maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@Rachaelisa's PR #28 was approved and merged by @codebestia.

🏆 @Rachaelisa: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (48 total points). Track your full progress on GrantFox.

👏 Great work, @Rachaelisa! Keep contributing to ShadeProtocol.