Add MTCL backend and multinode integration tests

[marcoSanti]

• Add MTCL remote backend with dynamic discovery, connection lifecycle management, asynchronous sends, and framed request/file transfers.
• Extend the backend API to send request metadata and file data as one ordered operation while preserving existing backend behavior.
• Harden remote request parsing and read handling, including malformed requests, zero-length reads, bounded byte counts, and correct file offsets.
• Replace the legacy integration test executable with split, map, and merge programs executed across three containers.
• Add Docker Compose multinode coverage to CI, improve failure log reporting, and make the test image configurable by base image.
• Add unit coverage for owned remote requests and compound backend sends.

[codecov]

Codecov Report

❌ Patch coverage is 26.51685% with 327 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.12%. Comparing base (ea6aab0) to head (848e695).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
capio/server/src/mtcl_backend.cpp	32.91%	44 Missing and 115 partials ⚠️
capio/tests/unit/server/src/remote_backend.cpp	0.00%	58 Missing ⚠️
capio/tests/unit/server/src/cli_parser.cpp	0.00%	29 Missing ⚠️
capio/server/include/remote/listener.hpp	25.71%	13 Missing and 13 partials ⚠️
capio/tests/unit/server/src/none_backend.cpp	0.00%	14 Missing ⚠️
capio/server/src/mpi_backend.cpp	0.00%	5 Missing and 8 partials ⚠️
capio/tests/unit/server/src/storage_manager.cpp	0.00%	11 Missing ⚠️
capio/tests/unit/server/src/main.cpp	0.00%	7 Missing ⚠️
capio/tests/unit/server/src/capio_file.cpp	0.00%	6 Missing ⚠️
capio/server/src/remote_request.cpp	87.50%	0 Missing and 2 partials ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #223      +/-   ##
==========================================
- Coverage   59.81%   55.12%   -4.69%     
==========================================
  Files          81       89       +8     
  Lines        3175     3951     +776     
  Branches     1973     2507     +534     
==========================================
+ Hits         1899     2178     +279     
- Misses        984     1134     +150     
- Partials      292      639     +347     

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[marcoSanti]

removed as this was duplicated (in signals.hpp it is already removed) and some times it was cause for a segfault during termination (double free)

[GlassOfWhiskey]

LGTM