Validates an authorization response (the query parameters delivered to the redirect URI) against its ConduitMcp.Client.OAuth.Pending, before the code goes anywhere near a token endpoint.

validate/3 runs these checks in order and stops at the first failure:

1. state equals the pending state, compared in constant time with Plug.Crypto.secure_compare/2; missing or different ⇒ :invalid_state.

2. Principal - the :principal the callback is completed under equals (===) the one the flow was started with ⇒ else :principal_mismatch.

3. Expiry - now is before expires_at ⇒ else :state_expired.

4. iss (RFC 9207 §2.4, OA-3), by exact string comparison with the recorded issuer and no normalization of case, slashes or ports:

   iss in the response	AS advertises iss support	Result
   present, equal	either	continue
   present, different	either	:iss_mismatch
   absent	true	:iss_missing
   absent	false / not advertised	continue

   A present iss is compared even when the authorization server never advertised the parameter. A key with a nil value counts as absent.

5. error - an error response becomes the matching typed reason (:access_denied, ... or :authorization_error for an unknown code), with the server's error, error_description and error_uri strings in :data.

6. code - returned wrapped in ConduitMcp.Client.Secret; absent ⇒ :missing_code.

Steps 1-4 come first so that a response from the wrong party, or for the wrong flow, surfaces nothing it carries: on any of those failures the error's :data is nil, and in particular a mismatched response's error_description is never passed through.

Replay

validate/3 is pure, so it accepts the same Pending and parameters as often as it is called. Single use of state is enforced by the host's store, which must delete the pending entry when it is looked up (see ConduitMcp.Client.OAuth.Pending).