types: fix OIDC identifier path traversal dropping subject

url.JoinPath resolves path-traversal segments like '..' and '.',
which silently drops the OIDC subject from the identifier. For
example, Iss='https://example.com' with Sub='..' produces
'https://example.com' — the subject is lost entirely. This causes
distinct OIDC users to receive colliding identifiers.

Replace url.JoinPath with simple string concatenation using a slash
separator. This preserves the subject literally regardless of its
content. url.PathEscape does not help because dots are valid URL
path characters and are not escaped.