6.0.0

Changed

• Adopts swift-api-client 5.0.0, swift-llm-client 5.0.0 and swift-http-transport 2.1.0. A 403 maps
  to LLMError.unauthorized alongside 401 — that case's own doc already covers both ("missing,
  wrong, or not allowed to reach this model"), neither is retryable, and both are fixed by changing
  the credential or the plan.
• The APIError.invalidResponse arm of the error mapping is gone. It was dead: upstream never
  constructed the case, so the condition always arrived as TransportError.invalidResponse inside
  .networkError, and the branch's "Invalid URL or response" message described something it
  never handled.

Changed

• Raised the swift-api-client pin to 5.0.0 and the swift-http-transport pin to 2.0.0. 4.0.0 of
  api-client cannot resolve alongside http-transport 2.x — its own pin bump did not land until
  5.0.0 — and 5.0.0 is also the release that boxes the injected transport in
  AnyStreamingTransport, which is what keeps APIClientImpl's existential transport working
  now that RetryingTransport is generic. Nothing this package names changes.
• mapAPIErrorToLLMError takes APIError's new shape. .forbidden joins .unauthorized on
  LLMError.unauthorized, which already means "the API key is missing, wrong, or not allowed
  to reach this model" and is what every provider contract here already returns for both 401
  and 403; neither is retryable and both are fixed by changing the credential or the plan
  behind it. .conflictingAuthHeader becomes invalidRequest, since the request is refused
  before it is sent.
• The invalidResponse arm is gone with the case. It was never reachable: APIError never
  constructed it, and the condition has always arrived as TransportError.invalidResponse
  inside .networkError, which is where it still arrives.