Improve tests for version package: add BuildVersionString coverage

Add TestBuildVersionString with 8 subtests covering all explicit-parameter
branches of BuildVersionString. Coverage of the function rises from 0% to
68%, and overall package coverage rises from 12% to 72%.

The remaining uncovered lines are the debug.ReadBuildInfo() fallback paths
(vcs.revision and vcs.time settings) which are only populated when the binary
is built with embedded VCS metadata and cannot be exercised in unit tests.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

internal/version/version_test.go

@@ -1,9 +1,11 @@
 package version
 
 import (
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestSet(t *testing.T) {
@@ -57,6 +59,63 @@ func TestSet(t *testing.T) {
 	}
 }
 
+func TestBuildVersionString(t *testing.T) {
+	t.Run("all fields provided", func(t *testing.T) {
+		got := BuildVersionString("v1.2.3", "abc1234", "2024-01-01")
+		assert.Equal(t, "v1.2.3, commit: abc1234, built: 2024-01-01", got)
+	})
+
+	t.Run("empty mainVersion uses dev prefix", func(t *testing.T) {
+		got := BuildVersionString("", "abc1234", "2024-01-01")
+		assert.Equal(t, "dev, commit: abc1234, built: 2024-01-01", got)
+	})
+
+	t.Run("mainVersion and gitCommit without buildDate", func(t *testing.T) {
+		got := BuildVersionString("v1.2.3", "abc1234", "")
+		// buildDate is empty so build info fallback may or may not add a date
+		assert.True(t, strings.HasPrefix(got, "v1.2.3, commit: abc1234"),
+			"result should start with version and commit: got %q", got)
+	})
+
+	t.Run("mainVersion and buildDate without gitCommit", func(t *testing.T) {
+		got := BuildVersionString("v1.2.3", "", "2024-01-01")
+		// gitCommit is empty so build info fallback may or may not add a commit
+		assert.True(t, strings.HasPrefix(got, "v1.2.3"),
+			"result should start with version: got %q", got)
+		assert.True(t, strings.HasSuffix(got, "built: 2024-01-01"),
+			"result should end with built date: got %q", got)
+	})
+
+	t.Run("only mainVersion", func(t *testing.T) {
+		got := BuildVersionString("v2.0.0", "", "")
+		// No explicit commit or date; build info fallback may add them
+		assert.True(t, strings.HasPrefix(got, "v2.0.0"),
+			"result should start with mainVersion: got %q", got)
+	})
+
+	t.Run("all empty uses dev", func(t *testing.T) {
+		got := BuildVersionString("", "", "")
+		assert.True(t, strings.HasPrefix(got, "dev"),
+			"result should start with 'dev' when mainVersion is empty: got %q", got)
+	})
+
+	t.Run("result is comma-separated", func(t *testing.T) {
+		got := BuildVersionString("v1.0.0", "deadbeef", "2025-06-01")
+		parts := strings.Split(got, ", ")
+		require.GreaterOrEqual(t, len(parts), 3, "should have at least 3 comma-separated parts")
+		assert.Equal(t, "v1.0.0", parts[0])
+		assert.Equal(t, "commit: deadbeef", parts[1])
+		assert.Equal(t, "built: 2025-06-01", parts[2])
+	})
+
+	t.Run("explicit gitCommit is not truncated", func(t *testing.T) {
+		longHash := "abcdef1234567890"
+		got := BuildVersionString("v1.0.0", longHash, "2024-01-01")
+		// When gitCommit is explicitly provided it is used as-is (no truncation)
+		assert.Contains(t, got, "commit: "+longHash)
+	})
+}
+
 func TestGet(t *testing.T) {
 	t.Run("returns default version", func(t *testing.T) {
 		original := gatewayVersion