fix: Fix unity build for book step

[a1q123456]

High Level Overview of Change

This PR adds a static check in src/libxrpl/tx/paths/BookStep.cpp to avoid the case where both TIn and TOut are XRPAmount.

Context of Change

Because unity build puts OfferStream.cpp and BookStep.cpp in the same translation unit, the compiler sees TOfferStreamBase<XRPAmount, XRPAmount> which further triggers the compiler error.

With this fix, we explicitly disallow this combination so that we get rid of the build error.

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[xrplf-ai-reviewer]

Had a look at this

One incorrect namespace string in the new UNREACHABLE tag — see inline.

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Review by ReviewBot 🤖

Review by Claude Opus 4.6 · Prompt: V14

[xrplf-ai-reviewer]

No issues.

Review by Claude Opus 4.6 · Prompt: V14

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.6%. Comparing base (a873250) to head (e28bb83).
⚠️ Report is 6 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #6942   +/-   ##
=======================================
  Coverage     81.6%   81.6%           
=======================================
  Files         1010    1010           
  Lines        75982   75972   -10     
  Branches      7632    7606   -26     
=======================================
- Hits         61986   61984    -2     
+ Misses       13996   13988    -8     

Files with missing lines	Coverage Δ
src/libxrpl/tx/paths/BookStep.cpp	95.9% <100.0%> (-<0.1%)	⬇️

... and 26 files with indirect coverage changes

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[xrplf-ai-reviewer]

No issues.

Review by Claude Opus 4.6 · Prompt: V15

[gregtatcam]

👍 LGTM