fix: Serialize MPToken UInt64 amounts as base-10 strings

[ckeshava]

Summary

The account_mptokens and account_mpt_issuances RPCs emitted MPToken amount fields as JSON numbers. Because many JSON parsers back numbers with IEEE-754 doubles, any value greater than 2^53 was silently rounded, so clients could not recover the exact on-ledger amount.

This change serializes the affected UInt64 amount fields as base-10 JSON strings, matching rippled's STUInt64::getJson behavior so Clio's output is consistent with the reference implementation:

• account_mptokens: mpt_amount, locked_amount
• account_mpt_issuances: maximum_amount, outstanding_amount, locked_amount

Changes

• Serialize the UInt64 amount fields via xrpl::STUInt64{field, value}.getJson(...) instead of inserting the raw integer into the JSON object, so the encoding matches rippled's string representation exactly.
• Add regression tests in both handlers asserting that large amounts (2^63 - 1, 2^53 + 1, and an odd value > 2^53) round-trip as exact strings.

Test plan

• New unit tests: RPCAccountMPTokensHandlerTest.LargeAmountsSerializedAsStrings and RPCAccountMPTokenIssuancesHandlerTest.LargeAmountsSerializedAsStrings.
• Existing MPToken handler tests updated to expect string-typed amount fields.

Notes

mpt_holders already serializes mpt_amount correctly via STUInt64::getJson; all other handlers that expose ledger UInt64 fields (e.g. vault_info, get_aggregate_price, gateway_balances) route through rippled's own serialization or STAmount, so they were already emitting strings and are unaffected.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.66%. Comparing base (52d28f8) to head (560501a).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3139   +/-   ##
========================================
  Coverage    82.65%   82.66%           
========================================
  Files          398      398           
  Lines        16118    16127    +9     
  Branches      8443     8450    +7     
========================================
+ Hits         13323    13331    +8     
  Misses        1674     1674           
- Partials      1121     1122    +1     

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[ckeshava]

/ai-reviewer