[BUG] DIF PE (ld_proof) holder always signs with auto-detected DID; explicit issuer_id in send-presentation is ignored

[jacksonriding]

Describe the Bug

When responding to a DIF Presentation Exchange request for a ld_proof-format credential, DIFPresFormatHandler.create_pres always calls DIFPresExchHandler.create_vp(..., is_holder_override=True). This forces the if self.is_holder or is_holder_override: branch inside create_vp, which unconditionally calls get_sign_key_credential_subject_id() to auto-detect the signing DID — completely ignoring self.pres_signing_did, i.e. any explicit issuer_id passed via POST /present-proof-2.0/records/{id}/send-presentation. For a did:key credential subject (the norm in --no-ledger setups), the auto-detection fails to select a signing key, so the resulting VP is returned with no proof at all, and the exchange completes with no error on the holder side. Separately, issuer_id also gets silently reset to None in create_pres unless presentation_definition is re-supplied in the same manual send-presentation call.

Steps to Reproduce

1. Run two ACA-Py agents with --no-ledger, connected via OOB + DID Exchange (did:peer).
2. On each agent, create a did:key (POST /wallet/did/create {"method":"key","key_type":"ed25519"}).
3. Issue a JSON-LD (ld_proof, Ed25519Signature2018) credential from agent A to agent B's did:key via Issue Credential 2.0.
4. From agent A, send a DIF PE presentation request (POST /present-proof-2.0/send-request) for that credential.
5. On agent B, respond via --auto-respond-presentation-request, or manually call POST /present-proof-2.0/records/{id}/send-presentation with {"dif": {"issuer_id": "<the did:key>"}}.

Expected Behavior

The presentation is signed using the credential's did:key subject (either auto-detected correctly, or using the explicitly supplied issuer_id), and verification on agent A succeeds (verified: true).

Actual Behavior

The submitted VP has no proof field. Agent A's verifier logs:
DIF-PRES verify_pres result: verified=False errors=[LinkedDataProofException('presentation must contain "proof"')]
No error or warning is logged on the holder (agent B) side — it returns 200 and completes the exchange as if nothing went wrong.

Environment

• ACA-Py version: py3.13-1.6-lts (ghcr.io/openwallet-foundation/acapy-agent:py3.13-1.6-lts)
• Python version: 3.13
• OS: Docker containers (Linux), orchestrated from Windows host

Additional Context

Reproduced consistently across multiple fresh wallets/connections while building an integration project with home assistant.

[jacksonriding]

Implemented in PR #4196

The fix preserves manual issuer_id, honours explicit pres_signing_did in holder mode, improves automatic local did:key discovery by skipping non-local candidates, and fails explicitly when holder signing cannot determine a local signing DID instead of returning an unsigned VP.

Validation: 74 affected DIF tests passed, 46 optional BBS tests skipped, Ruff passed, and git diff --check passed.