[Refactor] sdk/src/client.ts: centralize registry API error handling into apiGet

[shwetank-dev]

Problem

Every public API method in MpakClient (8 in total) repeats the same two-block error check after every fetchWithTimeout call:

if (response.status === 404) {
  throw new MpakNotFoundError(...);
}
if (!response.ok) {
  throw new MpakNetworkError(`Failed to ...: HTTP ${response.status}`);
}

This means adding a new error type (e.g. 429 rate limit, 401 unauthorized) requires touching every single method individually.

Proposed fix

Replace fetchWithTimeout with a private apiGet method that owns the full lifecycle of a registry API call — timeout, headers, error handling, and JSON parsing — and returns Promise<unknown>:

private async apiGet(
  url: string,
  init?: RequestInit & { notFoundKey?: string },
): Promise<unknown>

• If notFoundKey is provided and the response is 404, throw MpakNotFoundError(notFoundKey)
• If response is not ok, throw MpakNetworkError
• If response is ok, return response.json()

Callers then become a single line each:

return this.apiGet(url, { notFoundKey: name }) as Promise<BundleDetailResponse>;

Future error types are added in one place only.

Out of scope

Content downloads (downloadSkillContent, skill resolution) fetch arbitrary external URLs and return raw text — they are a different concern and not covered by this change.