Fix invalid 'for' attributes in agent test vectors

The 'for' field in agent objects must contain DIDs that reference actual
parties/agents, not string literals like 'merchant' or 'originator'.

Per TAIP-5: 'for' field should contain 'a DID or an array of DIDs of
another Agent or Party that this agent acts on behalf of'.

Fixed test vectors:
- payment-request/valid-direct-asset.json: 'merchant' → 'did:web:merchant.vasp'
- payment-request/valid-fiat-amount.json: 'merchant' → 'did:web:merchant.vasp'
- payment-request/invalid-missing-amount.json: 'merchant' → 'did:web:merchant.vasp'
- transfer/minimal.json: 'originator' → 'did:web:originator.vasp'

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: pelle

test-vectors/payment-request/invalid-missing-amount.json

@@ -25,7 +25,7 @@
         {
           "@id": "did:web:merchant.vasp",
           "role": "Merchant",
-          "for": "merchant"
+          "for": "did:web:merchant.vasp"
         }
       ]
     }

test-vectors/payment-request/valid-direct-asset.json

@@ -25,7 +25,7 @@
         {
           "@id": "did:web:merchant.vasp",
           "role": "Merchant",
-          "for": "merchant"
+          "for": "did:web:merchant.vasp"
         }
       ]
     }

test-vectors/payment-request/valid-fiat-amount.json

@@ -31,7 +31,7 @@
         {
           "@id": "did:web:merchant.vasp",
           "role": "Merchant",
-          "for": "merchant",
+          "for": "did:web:merchant.vasp",
           "policies": [
             {
               "@type": "RequirePresentation",

test-vectors/transfer/minimal.json

@@ -22,7 +22,7 @@
         {
           "@id": "did:web:originator.vasp",
           "role": "Originator",
-          "for": "originator"
+          "for": "did:web:originator.vasp"
         }
       ]
     }