Fix/mdoc sub document number

[grahamallen-1]

Summary

Use the mDL document number as the OIDC sub for mso_mdoc (mDL) presentations, instead of the enveloped credential id.

Problem

For an mso_mdoc presentation the credential is an EnvelopedVerifiableCredential whose credentialSubject.id is the data:application/mdl;base64,… URI — i.e. the entire mDL DeviceResponse. extractClaimsForIdToken uses that as sub, which is broken two ways:

• Non-deterministic. The DeviceResponse is regenerated every presentation (fresh device signature / session transcript / nonce), so sub differs on every login and an RP can never recognize a returning user.
• Oversized / leaks the whole mDL. The value is the full base64 mDL, far beyond the 255-ASCII-character limit OpenID Connect Core 1.0 §2 sets for sub, and it ship the entire credential to the RP as the subject identifier.

OpenID Connect Core 1.0 §2 (ID Token): sub is "a locally unique and never reassigned identifier … It MUST NOT exceed 255 ASCII characters in length" — https://openid.net/specs/openid-connect-core-1_0.html#IDToken

Fix

common/oidc.js: for an mso_mdoc credential, derive sub from org.iso.18013.5.1.document_number (a stable, per-license identifier already flattened into credentialSubject by buildMdocCredentialSubject). When the document number was not disclosed, or for non-mdoc credentials, fall back to the existing credentialSubject.id ?? id — so the change is strictly never-worse-than the current behavior. VC and hybrid flows are unaffected.

Tests

Added to test/unit/60-oidc.test.js:

• mso_mdoc → sub is the document number, not the base64 id
• mso_mdoc without a disclosed document number → falls back to credentialSubject.id
• hybrid flow (VC first) → sub is the VC subject, unchanged