refactor: Assorted small DID fixes

[mvadari]

High Level Overview of Change

This PR:

• Makes addSLE in DIDSet a static function, instead of a free function.
• Renames Attestation to Data everywhere (an artifact of a previous name for the field).
• Actually runs a set of tests that were not included in the run function of DID_test.

Context of Change

AI code review

Description:
The helper function addSLE is a free function defined in the xrpl namespace with external linkage, no declaration in any header, and no static keyword:

TER
addSLE(ApplyContext& ctx, std::shared_ptr<SLE> const& sle, AccountID const& owner)

This function is logically private to DIDSet.cpp. As written:

1. It pollutes the xrpl namespace and could conflict with similarly named functions in other TUs.
2. It creates a potential ODR (One Definition Rule) violation risk.
3. It can be accidentally called from other translation units that include the right headers.

The fix is to declare it static or move it into an anonymous namespace (as is done consistently throughout the rest of the codebase — see DeleteAccount.cpp which uses namespace { ... } for all its helpers).

API Impact

N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.8%. Comparing base (252c676) to head (1519fdf).
⚠️ Report is 34 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #6552   +/-   ##
=======================================
  Coverage     79.8%   79.8%           
=======================================
  Files          878     878           
  Lines        67893   67893           
  Branches      7550    7550           
=======================================
+ Hits         54210   54211    +1     
+ Misses       13683   13682    -1     

Files with missing lines	Coverage Δ
include/xrpl/protocol/Protocol.h	100.0% <ø> (ø)
src/libxrpl/tx/transactors/did/DIDDelete.cpp	100.0% <ø> (ø)
src/libxrpl/tx/transactors/did/DIDSet.cpp	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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