mssql, ci: fix Database CI mssql-driver-tests job (#26850)

* mssql: add tcc linux include path flag for unixODBC headers (fixes #26816)

tcc does not search /usr/include by default, causing compilation failures
when building the mssql module on Linux with the bundled tcc after
unixodbc-dev is installed. Add `#flag linux -I/usr/include` under a
`$if tinyc` guard so tcc can locate sql.h and sqlext.h.

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

* mssql: avoid DROP TABLE IF EXISTS to prevent FreeTDS protocol state pollution

DROP TABLE IF EXISTS on a non-existent table causes SQL Server to send an
informational TDS token. FreeTDS may not drain this token before the
statement handle is freed, leaving the connection in a state where the next
SQLAllocHandle(STMT) returns a handle that immediately gives SQL_INVALID_HANDLE
on SQLExecDirect (no ODBC diagnostics). Replace with IF OBJECT_ID() IS NOT NULL
DROP TABLE, which only sends a done token and avoids the spurious info message.

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

* ci: skip GPG dearmor if microsoft-prod.gpg already exists on runner

Ubuntu 24.04 GitHub Actions runners ship with
/usr/share/keyrings/microsoft-prod.gpg pre-installed. The unconditional
`gpg --dearmor -o ...` fails with "File exists" because the file is
already present. Guard the step so it only runs when the file is absent.

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

---------

Co-authored-by: Richard Wheeler <18647491+PythonWillRule@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 3 people

.github/workflows/db_ci.yml

@@ -96,7 +96,7 @@ jobs:
       - name: Install ODBC dependencies
         run: |
           .github/workflows/disable_azure_mirror.sh
-          curl -fsSL https://packages.microsoft.com/keys/microsoft.asc | sudo gpg --batch --no-tty --dearmor -o /usr/share/keyrings/microsoft-prod.gpg
+          [ -f /usr/share/keyrings/microsoft-prod.gpg ] || curl -fsSL https://packages.microsoft.com/keys/microsoft.asc | sudo gpg --batch --no-tty --dearmor -o /usr/share/keyrings/microsoft-prod.gpg
           curl -fsSL https://packages.microsoft.com/config/ubuntu/24.04/prod.list | sudo tee /etc/apt/sources.list.d/mssql-release.list
           ./v retry -- sudo apt update
           sudo ACCEPT_EULA=Y apt install --quiet -y msodbcsql18 unixodbc-dev

vlib/db/mssql/_cdef_nix.c.v

@@ -13,5 +13,11 @@ $if $pkgconfig('odbc') {
 	#flag darwin -L/opt/local/lib
 }
 
+$if tinyc {
+	// tcc does not search system include paths by default; add /usr/include so
+	// that it can find the unixODBC headers (sql.h, sqlext.h) on Linux.
+	#flag linux -I/usr/include
+}
+
 #include <sql.h>
 #include <sqlext.h>

vlib/db/mssql/mssql_test.v

@@ -35,9 +35,9 @@ fn test_connection_and_query() {
 		conn.close()
 	}
 
-	conn.query('drop table if exists vlang_mssql_test') or {}
+	conn.query("if object_id('vlang_mssql_test', 'U') is not null drop table vlang_mssql_test") or {}
 	defer {
-		conn.query('drop table if exists vlang_mssql_test') or {}
+		conn.query("if object_id('vlang_mssql_test', 'U') is not null drop table vlang_mssql_test") or {}
 	}
 
 	create_result := conn.query('create table vlang_mssql_test (