GH-50560: [C++][FlightRPC][ODBC] Fix SQLDescribeCol column_size/decimal_digits width

[vikrantpuppala]

Rationale for this change

SQLDescribeCol returned uninitialized garbage in the upper bytes of its
column_size (ColumnSizePtr) output for numeric / decimal / integer / float
columns.

The numeric path read SQL_DESC_PRECISION — stored internally as a
SQLSMALLINT (2 bytes) — directly into the caller's SQLULEN* column_size_ptr
(8 bytes). The GetAttribute template that ultimately performs the write copies
exactly sizeof(T) bytes (here T = SQLSMALLINT, so 2 bytes) and ignores the
claimed buffer_length. The result: only the low 2 bytes of the 8-byte
SQLULEN were written and the upper 6 bytes were left uninitialized. Any
application reading the full column_size saw a wildly incorrect value.

Two adjacent decimal_digits sites (SQL_DESC_SCALE for exact-numeric columns,
SQL_DESC_PRECISION for datetime/interval columns) passed sizeof(SQLULEN) as
the buffer size even though decimal_digits_ptr is a SQLSMALLINT*. Those did
not corrupt memory (the write width matched the 2-byte target), but the size
argument was wrong and misleading.

Introduced by GH-47724 (#48052).

What changes are included in this PR?

• Numeric column_size path: read SQL_DESC_PRECISION into a local
  SQLSMALLINT and widen-assign it into *column_size_ptr, so all 8 bytes of
  the SQLULEN output are written.
• decimal_digits scale and datetime-precision paths: pass
  sizeof(SQLSMALLINT) (the true target width) instead of sizeof(SQLULEN).
• Added describe_col_test.cc with parameterized (mock + remote) tests that
  pre-fill column_size with an all-ones sentinel, call SQLDescribeCol on a
  DECIMAL column and a TIMESTAMP column, and assert the full 8-byte
  column_size is a sane small value (i.e. the upper bytes were overwritten),
  plus sane decimal_digits. Without the fix these fail because the sentinel's
  upper bytes survive the short write.

Are these changes tested?

Yes — new gtest cases in
cpp/src/arrow/flight/sql/odbc/tests/describe_col_test.cc, wired into the ODBC
test CMakeLists.txt.

Are there any user-facing changes?

Yes. SQLDescribeCol now returns a correct column_size for numeric/decimal
columns instead of a value with uninitialized high bytes. This is a bug fix in
externally-visible ODBC behavior; no API signatures change.

• GitHub Issue: [C++][FlightRPC][ODBC] SQLDescribeCol writes only 2 of 8 bytes of column_size for numeric/decimal columns #50560

[github-actions]

⚠️ GitHub issue #50560 has been automatically assigned in GitHub to PR creator.

[lidavidm]

This PR contains a "Critical Fix".

I think we reserve this for things that would be a release blocker, FWIW.

CC @justing-bq for review

[Copilot]

Pull request overview

Fixes incorrect SQLDescribeCol output widths in the Flight SQL ODBC driver so callers receive fully-initialized column_size for numeric/decimal types (and correct buffer-size arguments for decimal_digits), addressing GH-50560 and preventing nondeterministic garbage in upper bytes.

Changes:

• In SQLDescribeCol, reads SQL_DESC_PRECISION into a SQLSMALLINT and widens into SQLULEN to avoid short writes into column_size.
• Corrects decimal_digits GetField buffer sizes to sizeof(SQLSMALLINT) for scale / datetime precision paths.
• Adds a new gtest file to validate column_size is fully overwritten (and wires it into the ODBC test CMake target).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
cpp/src/arrow/flight/sql/odbc/tests/describe_col_test.cc	Adds regression tests intended to catch short writes into SQLULEN column_size and validate decimal_digits.
cpp/src/arrow/flight/sql/odbc/tests/CMakeLists.txt	Includes the new describe_col_test.cc in the ODBC test sources.
cpp/src/arrow/flight/sql/odbc/odbc_api.cc	Fixes write-width mismatch for numeric column_size and corrects buffer-size arguments for decimal_digits GetField calls.

[vikrantpuppala]

@lidavidm Dropped the "Critical Fix" wording from the description