fix(gotrue): replace assert with AuthException for PKCE asyncStorage check

[sanadidari]

Fixes #1319

Problem

When flowType is AuthFlowType.pkce (the default) and no asyncStorage
is provided, calling getOAuthSignInUrl() or getLinkIdentityUrl() crashes
in release mode with a cryptic Null check operator used on a null value
error. The existing assert() only runs in debug builds and is silently
stripped in release.

Changes

• _getUrlForProvider: replace assert + _asyncStorage! with an explicit
  null check that throws a descriptive AuthException in all build modes.
• getLinkIdentityUrl: add null check on res['url'] before casting,
  throwing a descriptive AuthException if the provider returns no URL.

Before / After

// Before (crashes silently in release)
assert(_asyncStorage != null, 'You need to provide asyncStorage...');
await _asyncStorage!.setItem(...);

// After (throws clear error in all build modes)
final storage = _asyncStorage;
if (storage == null) {
  throw AuthException('asyncStorage is required for PKCE flow...');
}
await storage.setItem(...);

[Vinzent03]

I think a better solution for this is to add an assert statement in the constructor of the gotrue client. But that is a breaking change and therefore add it in v3 #1278 of the library.