Add remote authorization server indication to OAuth2 error messages

[reinkrul]

Summary

Closes #4096

When the local Nuts node acts as a client and the remote OAuth2 authorization server returns an error, the error was propagated as-is to the caller (e.g. EHR), making it impossible to tell whether the error originated locally or remotely.

• Added RemoteOAuthError wrapper type in auth/oauth/error.go that prefixes error messages with "remote authorization server: "
• Updated HTTPClient.AccessToken() and HTTPClient.doRequest() to wrap parsed remote OAuth2 errors with RemoteOAuthError
• Fixed PresentationDefinition() to use errors.As instead of a direct type assertion (needed after wrapping)
• RemoteOAuthError implements HTTPStatusCodeError (delegates to inner error) and Unwrap() so errors.As still finds the underlying OAuth2Error

Before:

{"detail": "invalid_request - unable to get subject DID from VC: credential subjects have no ID", "status": 400, "title": "RequestServiceAccessToken failed"}

After:

{"detail": "remote authorization server: invalid_request - unable to get subject DID from VC: credential subjects have no ID", "status": 400, "title": "RequestServiceAccessToken failed"}

Test plan

• Unit tests added for RemoteOAuthError in auth/oauth/error_test.go
• Existing tests in auth/client/iam/client_test.go and openid4vp_test.go updated to assert RemoteOAuthError wrapping
• All unit tests pass (go test ./...)
• OAuth flow e2e tests pass

🤖 Generated with Claude Code

[qltysh]

Coverage Impact

⬆️ Merging this pull request will increase total coverage on master by 0.03%.

Modified Files with Diff Coverage (2)

Rating	File	% Diff	Uncovered Line #s
	auth/oauth/error.go	100.0%
	auth/client/iam/client.go	100.0%
	Total	100.0%

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