gRPC: lightningd: Fix empty description field in listpays & listsendpays

[erdoganishe]

Not ready to merge.

Fixes #7973.

This PR fixes the description field being returned as empty in gRPC responses for listpays and listsendpays.
The field already existed, but was not being populated correctly in these gRPC paths.

After this change, the description is correctly propagated and returned for completed payments.

Examples:

$ l1-cli listsendpays                    
{
   "payments": [
      {
         "created_index": 1,
         "id": 1,
         "payment_hash": "8aeb87ea41d9a116336c8051a42484e01f7e366b51a6d66141226c794fcea817",
         "groupid": 1,
         "updated_index": 1,
         "destination": "02270cc33bf20e7f7a84ee2efb7343926f6d0c341f7f3a45f4d9ca51647feaa1c4",
         "amount_msat": 1000,
         "amount_sent_msat": 1000,
         "created_at": 1766404635,
         "completed_at": 1766404635,
         "status": "complete",
         "payment_preimage": "32b6f55f6eec622b917f7495e739818523d8fa42cade6e81a05a17154d12af6e",
         "bolt11": "lnbcrt10n1p55jv0ysp5uwls677d6nad0krd7265kmwsaqmzgqg5kkps9gz3yqkqj6lr436qpp53t4c06jpmxs3vvmvspg6gfyyuq0hudnt2xndvc2pyfk8jn7w4qtsdq5w3jhxapqwpshjmt9de6qxqyjw5qcqp2fp4p0367fw5vnsq80lyvqv753rhd07e2rfkce204gmh0wyc6u7rzu4hq9qxpqysgq0lyg3e87e7lzxs0r5hyfvztlv24t2wfd0n6j23rajnn98tnl3h8n2u25nn4nywqjs4ze5wv35ch9ccl7xurlsxj052erj3hlnqwve9sp89u92x",
         "description": "test payment"
      }
   ]
}

l1-cli listpays                        
{
   "pays": [
      {
         "bolt11": "lnbcrt10n1p55jv0ysp5uwls677d6nad0krd7265kmwsaqmzgqg5kkps9gz3yqkqj6lr436qpp53t4c06jpmxs3vvmvspg6gfyyuq0hudnt2xndvc2pyfk8jn7w4qtsdq5w3jhxapqwpshjmt9de6qxqyjw5qcqp2fp4p0367fw5vnsq80lyvqv753rhd07e2rfkce204gmh0wyc6u7rzu4hq9qxpqysgq0lyg3e87e7lzxs0r5hyfvztlv24t2wfd0n6j23rajnn98tnl3h8n2u25nn4nywqjs4ze5wv35ch9ccl7xurlsxj052erj3hlnqwve9sp89u92x",
         "description": "test payment",
         "destination": "02270cc33bf20e7f7a84ee2efb7343926f6d0c341f7f3a45f4d9ca51647feaa1c4",
         "payment_hash": "8aeb87ea41d9a116336c8051a42484e01f7e366b51a6d66141226c794fcea817",
         "status": "complete",
         "created_at": 1766404635,
         "completed_at": 1766404635,
         "preimage": "32b6f55f6eec622b917f7495e739818523d8fa42cade6e81a05a17154d12af6e",
         "amount_msat": 1000,
         "amount_sent_msat": 1000,
         "created_index": 1,
         "updated_index": 1
      }
   ]
}

No schema change is required: the description field already existed in both the database and RPC definitions.
The fix ensures the field is correctly populated and forwarded in gRPC responses.
Existing SQL tests pass without modification, since description is an additional optional field and does not affect existing column expectations.