fix: use U256 return value for asset volume

[dandanlen]

Pull Request

Closes: PRO-1277

Checklist

Please conduct a thorough self-review before opening the PR.

• I am confident that the code works.
• I have updated documentation where appropriate.

Summary

Ensures we return hex-encoded amounts from broker api.
Improved error messages.

Examples:

broker_withdraw_fees success:

{
  "jsonrpc": "2.0",
  "result": {
    "tx_hash": "0x3fc8762e54d9eada666adb8adfabe18099611e62544751fff5b5f45a8a8db81d",
    "egress_id": [
      "Ethereum",
      93
    ],
    "egress_amount": "0xa68859da017bcdc",
    "egress_fee": "0x77a10",
    "destination_address": "0xabababababababababababababababababababab"
  },
  "id": 1
}

broker_withdraw_fees error:

{
  "jsonrpc": "2.0",
  "error": {
    "code": -32000,
    "message": "Module error ‘NoFundsAvailable‘ from pallet ‘Swapping‘: ‘The withdrawal is not possible because not enough funds are available.‘"
  },
  "id": 1
}

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.

Project coverage is 71%. Comparing base (4583957) to head (8c984bb).

Files	Patch %	Lines
api/lib/src/lib.rs	0%	10 Missing ⚠️
api/lib/src/lp.rs	0%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #4986    +/-   ##
======================================
- Coverage     71%     71%    -0%     
======================================
  Files        429     429            
  Lines      74082   73915   -167     
  Branches   74082   73915   -167     
======================================
- Hits       52931   52732   -199     
- Misses     18315   18345    +30     
- Partials    2836    2838     +2     

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