GH-49552: [C++][FlightRPC][ODBC] Enable ODBC test build on Linux

[justing-bq]

Rationale for this change

The test suite needs to be updated so it will build on Linux in addition to Windows & Mac.
Resolves #49552

What changes are included in this PR?

Miscellaneous changes to get the tests building on Linux.

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

• GitHub Issue: [C++][FlightRPC][ODBC] Enable ODBC test build on Linux #49552

[github-actions]

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See also:

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[github-actions]

⚠️ GitHub issue #49552 has been automatically assigned in GitHub to PR creator.

[justing-bq]

@lidavidm @kou
Please take a look when you have a chance.

[Copilot]

Pull request overview

Enables building the Flight SQL ODBC C++ test suite on Linux by removing the platform gate in CMake and updating tests/helpers to better handle platform differences in SQLWCHAR and wide-string handling.

Changes:

• Always add the ODBC tests subdirectory when ARROW_BUILD_TESTS is enabled (including Linux).
• Refactor many tests to avoid relying on L""/wcslen assumptions and to use helper macros/utilities for SQLWCHAR.
• Adjust CI to exclude running the ODBC test binary on Linux while still building it.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
cpp/src/arrow/flight/sql/odbc/tests/type_info_test.cc	Switches expected column name handling and replaces NULL sentinel usage with numeric values.
cpp/src/arrow/flight/sql/odbc/tests/tables_test.cc	Updates SQLWCHAR parameter initialization and column-name comparisons for portability.
cpp/src/arrow/flight/sql/odbc/tests/statement_test.cc	Introduces ASSIGN_SQLWCHAR_ARR_AND_LEN usage and adjusts string comparisons/conversions.
cpp/src/arrow/flight/sql/odbc/tests/statement_attr_test.cc	Uses the new SQLWCHAR assignment helper macro for query strings.
cpp/src/arrow/flight/sql/odbc/tests/odbc_test_suite.h	Adds Linux-specific SQLWCHAR assignment macros and declares new conversion helpers.
cpp/src/arrow/flight/sql/odbc/tests/odbc_test_suite.cc	Implements new SQLWCHAR length/conversion helpers and tweaks DSN registration call.
cpp/src/arrow/flight/sql/odbc/tests/errors_test.cc	Avoids constructing std::wstring directly from SQLWCHAR* by converting to UTF-8 first.
cpp/src/arrow/flight/sql/odbc/tests/connection_test.cc	Makes output buffers consistently zero-initialized.
cpp/src/arrow/flight/sql/odbc/tests/connection_info_test.cc	Renames helper and converts many wide-string assertions to use ConvertToWString.
cpp/src/arrow/flight/sql/odbc/tests/columns_test.cc	Updates SQLWCHAR parameter handling and metadata assertions for Linux builds.
cpp/src/arrow/flight/sql/odbc/CMakeLists.txt	Removes Windows/macOS-only gating so tests are built on Linux too.
ci/scripts/cpp_test.sh	Excludes arrow-flight-sql-odbc-test from Linux test execution.

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[Copilot]

WriteDSN converts the driver name to std::wstring and then reinterpret_casts w_driver.c_str() to LPCWSTR. On Linux, the driver manager expects UTF-16 SQLWCHAR (see system_dsn.cc ConvertToSQLWCHAR), so casting wchar_t* (UTF-32) to LPCWSTR is likely incorrect and can mis-encode the driver name or cause misaligned reads. Convert the UTF-8 driver string to the platform’s expected SQLWCHAR representation before calling RegisterDsn (e.g., UTF8StringToUTF16 on Linux, UTF8ToWideString elsewhere), and pass the resulting buffer pointer without reinterpret_casting between incompatible character types.

[alinaliBQ]

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