Fix ROOT blackboard export in Groot2 publisher

[facontidavide]

Summary

Rebased and cleaned-up version of #1132 by @magic-alt (original authorship preserved on the commit).

When a subtree's blackboard is backed by a separate root blackboard — e.g. created via Blackboard::create(external_root) to expose @ globals — the Groot2 publisher never exported those root entries, so they were invisible in Groot2. This exports the root blackboard under the reserved name ROOT whenever a requested subtree resolves to a distinct root blackboard.

• Deduplicate the ROOT payload when several subtrees share the same root blackboard.
• Return the dump as Expected<> and send an error reply for ambiguous requests: a subtree literally named ROOT, or a request spanning multiple distinct root blackboards.
• Enable /utf-8 for behaviortree_cpp_test on MSVC so the existing Unicode name-validation tests build on Windows.

Closes #1130.

What changed vs #1132

#1132 was written against an older master, before the Groot2 thread-safety refactor (#1165) reshaped serverLoop/generateBlackboardsDump. This branch:

• Re-threads the new Expected<> return + error-reply path through the refactored serverLoop (the conflicting hunk).
• Moves the new tests out of gtest_loggers.cpp and into gtest_groot2_publisher_integration.cpp, where the other real-ZMQ Groot2 tests live — so they are gated BTCPP_GROOT_INTERFACE AND NOT WIN32 like their siblings (real-ZMQ tests are intentionally not run on Windows, see the note in tests/CMakeLists.txt).
• Reuses the shared Groot2Test::makePublisher / Client helpers instead of hand-rolling a ZMQ client, hardcoded ports, and a fixed sleep.

Design note

ROOT is exported for any requested subtree whose blackboard has a parent (every non-root subtree does, since Blackboard::create(parent) links the chain). So even plain multi-subtree trees with no external root now show a ROOT entry duplicating the main tree's blackboard. This matches the approach in #1130; if we'd rather expose ROOT only for genuinely external root blackboards, that's a small tweak to the needs_exported_root condition — happy to adjust.

Test plan

• cmake -S . -B build -DBTCPP_GROOT_INTERFACE=ON && cmake --build build — clean, no warnings.
• Full suite: 504/504 pass; Groot2PublisherIntegration.* 7/7 (2 existing + 5 new blackboard tests).
• BTCPP_GROOT_INTERFACE=OFF build: compiles clean, Groot2 tests correctly excluded.
• clang-format-21 clean on all touched files.

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
5 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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