fix: subscription webhook delivery stalling on HTTP errors

[sublimator]

Summary

• HTTPClient EOF completion leak: handleData logged "Complete." on EOF but never called invokeComplete(), leaking sockets until the 30s deadline timer fired
• RPCSub unbounded concurrency: sendThread() fired all queued events as concurrent async HTTP connections with no flow control, exhausting FDs

Changes

HTTPClient (src/libxrpl/net/HTTPClient.cpp)

EOF path in handleData now commits remaining response bytes and invokes the completion callback, matching the non-EOF path.

RPCSub (src/xrpld/rpc/detail/RPCSub.cpp)

• Batched dispatch: up to 32 events per batch on a local io_context with .run() for flow control (same pattern as rpcClient())
• Queue cap: 16384 events max, drops with warning log including seq and endpoint (consumers detect gaps via seq field)
• Catch-all exception handler

Cleanup

• Removed io_context parameter from make_RPCSub signature, constructor, and call site
• Removed boost/asio/io_context.hpp include from RPCSub.h

Upstream issue

#6341

Test plan

• Build succeeds
• Existing tests pass

[codecov]

Codecov Report

❌ Patch coverage is 4.16667% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.9%. Comparing base (db2734c) to head (7b4720b).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/xrpld/rpc/detail/RPCSub.cpp	4.8%	20 Missing ⚠️
src/libxrpl/net/HTTPClient.cpp	0.0%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #6344     +/-   ##
=========================================
- Coverage     79.9%   79.9%   -0.0%     
=========================================
  Files          840     840             
  Lines        65483   65494     +11     
  Branches      7251    7260      +9     
=========================================
- Hits         52332   52317     -15     
- Misses       13151   13177     +26     

Files with missing lines	Coverage Δ
src/xrpld/rpc/handlers/Subscribe.cpp	90.5% <ø> (-0.1%)	⬇️
src/libxrpl/net/HTTPClient.cpp	70.7% <0.0%> (-1.2%)	⬇️
src/xrpld/rpc/detail/RPCSub.cpp	37.1% <4.8%> (-8.8%)	⬇️

... and 9 files with indirect coverage changes

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[sublimator]

CI needs approval to run on the latest commits (added RPCSub + HTTPClient tests for coverage). Could a maintainer approve the workflow runs?