C++ client: add thread-safe SessionPool, enable RPC compression, and harden buffers

[SpriCoder]

Summary

This PR improves the C++ client (iotdb-client/client-cpp) along three axes — concurrency, read/write performance, and safety/correctness. It is split into four logically separate commits.

1. Thread-safe SessionPool (new feature)

A single Session is not safe to use from multiple threads concurrently (it owns one Thrift transport/client). Previously the C++ client had no pooling, forcing callers to either serialize all requests through one session or hand-roll connection management.

• New SessionPool + SessionPoolBuilder lend each Session to exactly one borrower at a time and reclaim it afterwards, so many application threads can share a bounded set of connections without external locking.
• getSession() returns an RAII PooledSession handle that returns the session to the pool on scope exit and works with any Session method; a generic execute([](Session&){...}) plus convenience wrappers (insertTablet, insertRecord, executeQueryStatement, …) cover common operations.
• New sessions are constructed outside the pool lock so a handshake never blocks other borrowers; getSession() blocks up to a configurable timeout when the pool is exhausted.
• Query results are returned as a PooledSessionDataSet that keeps the session leased until the result set is fully read — necessary because SessionDataSet lazily fetches further blocks over the same connection. Sessions that raise IoTDBConnectionException are evicted instead of recycled.

2. Enable RPC compression that was silently ignored (performance)

Session::open(bool enableRPCCompression) and the builder's enableRPCCompression flag were never propagated to SessionConnection, whose flag was hardcoded to false, so the compact Thrift protocol never took effect. The flag is now threaded from the builder/open() into both the data SessionConnection and the node-discovery NodesSupplier client, reducing bytes on the wire for both reads and writes.

3. Safety / correctness hardening

• Tablet::addValue formatted out-of-range diagnostics with sprintf into a fixed 100-byte stack buffer; switched to snprintf bounded by sizeof(buffer) to remove the overflow risk.
• MyStringBuffer::putOrderedByte used str.assign on big-endian hosts, which overwrote the whole buffer on every numeric write and corrupted previously serialized content; changed to str.append to match the little-endian path.

Commits

1. Wire RPC compression flag through Session to its connections
2. Use snprintf for Tablet bounds-check error messages
3. Append big-endian bytes in MyStringBuffer instead of overwriting
4. Add thread-safe SessionPool to the C++ client

Test plan

• All changed/new sources compile cleanly against the project's Thrift/Boost headers (g++ -std=c++11 -fsyntax-only, 0 errors); new code conforms to the repo .clang-format (2-space, 100-col).
• Run the C++ client integration tests against a live IoTDB on 127.0.0.1:6667 (session_tests), including the new [sessionPool] cases: basic borrow/insert/query, concurrent writers, and exhaustion-timeout. These require a running server and were not executed in the authoring environment.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

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

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 40.82%. Comparing base (0848379) to head (6e635c3).

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #17800   +/-   ##
=========================================
  Coverage     40.82%   40.82%           
+ Complexity     2611     2610    -1     
=========================================
  Files          5184     5184           
  Lines        350965   350965           
  Branches      44895    44895           
=========================================
+ Hits         143276   143279    +3     
+ Misses       207689   207686    -3     

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