fix: replace type assertions with proper HttpError property

[Copilot]

Addresses type safety violations in RetryableHttpClient where retryAfterHeader was attached to HttpError instances via type assertions (error as any).

Changes

• HttpError class: Added optional retryAfterHeader?: string | null parameter to constructor
• Error creation (line 384-393): Pass header value in constructor instead of post-hoc assignment
• Error consumption (line 324): Access typed property instead of using type assertion

Before/After

// Before: Type assertion bypass
const error = new HttpError(message, status, statusText, body, retryable);
if (response.status === 429) {
  (error as any).retryAfterHeader = response.headers.get("Retry-After");
}

// Later access also unsafe
const retryAfter = parseRetryAfter((error as any).retryAfterHeader || null);

// After: Type-safe throughout
const error = new HttpError(
  message, 
  status, 
  statusText, 
  body, 
  retryable,
  response.status === 429 ? response.headers.get("Retry-After") : undefined
);

// Later access is type-safe
const retryAfter = parseRetryAfter(error.retryAfterHeader ?? null);

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[angelo-hub]

LGTM