feat: add PEX support for OID4VP presentations

[romitshah02]

Description

Adds DIF Presentation Exchange (PEX) v2.0 support to oid4vc-service's OID4VP verifier flow, as an alternative query language to the existing DCQL-only implementation. Enables live VP round trips against wallets (e.g. Inji) that implement PEX but not DCQL. Also fixes real holder-binding and key-selection bugs surfaced by testing against a live wallet, and adds JsonWebSignature2020 proof verification + local did:jwk resolution to credentials-service.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Refactor / Technical Debt
• Test case addition/update

Microservice(s) Affected

• oid4vc-service
• credentials-service

Changes

oid4vc-service — PEX support:

• New PexService (pex.service.ts): full DIF PEX v2.0 evaluator — input_descriptors[].constraints.fields path/filter matching, format restriction, submission_requirements (pick/all, from/from_nested)
• New shared jsonpath.util.ts: minimal JSONPath subset (dot/bracket segments, numeric indices) used by both PexService and descriptor-map resolution
• oid4vp.service.ts: VpTxn.queryMode: 'dcql' | 'pex' — request accepts presentation_definition as a mutually-exclusive alternative to dcql_query; submitResponse() branches per mode; per-credential verification extracted into a shared verifyPresentationEntry() used by both DCQL and PEX loops
• resolveDescriptorMapEntry(): resolves descriptor_map[].path / path_nested chains against the wallet's vp_token
• normalizePexVpToken(): absorbs PEX's vp_token shape (raw presentation, not always JSON, unlike DCQL's always-object shape)

Bug fixes (found via live wallet testing):

• resolveDescriptorMapEntry() no longer descends into path_nested when the outer format is ldp_vp — descending discarded the outer VP's holder-binding proof and exposed only the embedded VC's static issuance-time proof
• Holder DID comparison strips the verification-method #fragment (did:jwk:...#0 vs the bare DID) before comparing to credentialSubject.id
• oid4vci.service.ts: issuer metadata now advertises EdDSA in proof_types_supported.jwt.proof_signing_alg_values_supported (previously ES256 only) — some wallets hardcode Ed25519 for presentation-signing independent of their issuance-time key selection, so the issuer must advertise EdDSA for that key to ever be selectable at issuance

credentials-service:

• verifyCredential() dispatches to a new verifyPresentation() path when type includes VerifiablePresentation, instead of only ever treating the body as a bare VC
• New jws2020.util.ts: hand-rolled JsonWebSignature2020 Linked-Data proof verifier (URDNA2015 canonicalization + detached JWS per RFC7797) — no installed suite covers this proof type
• New self-contained-did.util.ts: local did:jwk resolution (public key is the DID's own base64url JWK payload, no network lookup)

Notes

• No new runtime dependency — PEX evaluator is hand-rolled, mirroring DcqlService's existing non-library approach
• No schema/DB change, no new top-level API route — presentation_definition is an additive, mutually-exclusive alternative field on the existing POST /vp/request body
• Three behavior changes affect pre-existing DCQL callers, not just new PEX requests (detailed in the design doc): QR/qr_data link is now returned by-value instead of always by-reference; issued VC @context now rewrites internalUrl → publicUrl for all issuance; ldp_vc plain-VP-object holder binding is stricter (credentialSubject.id must equal the VP's holder, not merely exist)