refactor(grpc): use public solve_lp/mip_remote in integration tests

Replace the internal grpc_client_t (and the interim grpc_cpp_client_t
wrapper) with the already-public solve_lp_remote() / solve_mip_remote()
API in GRPC_INTEGRATION_TEST. Each fixture sets CUOPT_REMOTE_HOST /
CUOPT_REMOTE_PORT via setenv() so the public functions can reach the
per-test server without any internal client class.

Changes:
- Delete grpc_cpp_client_t (grpc_cpp_client.hpp, grpc_client_types.hpp,
  grpc_cpp_client.cpp) — unnecessary abstraction
- Revert grpc_client.hpp and grpc_client_env.hpp to their pre-PR state
- Remove src/grpc/client from GRPC_INTEGRATION_TEST include paths
- Rewrite grpc_integration_test.cpp to use only the public API:
  • ServerProcess::wait_for_ready uses raw gRPC channel connectivity
  • GrpcIntegrationTestBase sets/clears env vars in SetUp/TearDown
  • TLS / mTLS tests use CUOPT_TLS_* env vars (cert file paths)
  • ChunkedUploadTests sets CUOPT_MAX_MESSAGE_BYTES=4096 to trigger
    the chunked transport path for medium-sized problems
  • QCQP end-to-end tests kept (functional value independent of path)
  • ChunkValidationTests kept unchanged (raw gRPC stub)
- Remove tests that require the internal async lifecycle API:
  submit_lp/mip, check_status, cancel_job, delete_job, stream_logs,
  get_result, debug_log_callback, log_callback, incumbent_callback

This eliminates any static-library linkage risk and makes the test
binary depend only on the public cuopt shared library.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ramakrishnap-nv

cpp/tests/linear_programming/grpc/CMakeLists.txt

@@ -99,7 +99,6 @@ target_include_directories(GRPC_INTEGRATION_TEST
     PRIVATE
     "${CUOPT_SOURCE_DIR}/include"
     "${CUOPT_SOURCE_DIR}/src/grpc"
-    "${CUOPT_SOURCE_DIR}/src/grpc/client"
     "${CUOPT_TEST_DIR}"
     "${CMAKE_BINARY_DIR}"  # For generated protobuf headers
 )