build(deps): bump cloud.google.com/go/longrunning from 0.9.0 to 0.12.0

Update cloud.google.com/go/longrunning from 0.9.0 to 0.12.0.

Summary:
- Bumps the direct longrunning dependency to v0.12.0 after the Spanner dependency update.
- Rebased diff is limited to one go.mod version line and the corresponding go.sum checksums.
- No code changes are required in spanner-mycli.

Validation:
- CI lint, test, and readme-sync passed on the rebased PR head.
- Unresolved review threads: 0.

Author: dependabot[bot]

go.mod

@@ -4,7 +4,7 @@ go 1.25.0
 
 require (
 	cloud.google.com/go v0.123.0
-	cloud.google.com/go/longrunning v0.9.0
+	cloud.google.com/go/longrunning v0.12.0
 	cloud.google.com/go/spanner v1.91.0
 	cloud.google.com/go/storage v1.56.1
 	github.com/MakeNowJust/heredoc/v2 v2.0.1

go.sum

@@ -1386,8 +1386,8 @@ cloud.google.com/go/longrunning v0.5.12/go.mod h1:S5hMV8CDJ6r50t2ubVJSKQVv5u0rmi
 cloud.google.com/go/longrunning v0.6.0/go.mod h1:uHzSZqW89h7/pasCWNYdUpwGz3PcVWhrWupreVPYLts=
 cloud.google.com/go/longrunning v0.6.1/go.mod h1:nHISoOZpBcmlwbJmiVk5oDRz0qG/ZxPynEGs1iZ79s0=
 cloud.google.com/go/longrunning v0.6.2/go.mod h1:k/vIs83RN4bE3YCswdXC5PFfWVILjm3hpEUlSko4PiI=
-cloud.google.com/go/longrunning v0.9.0 h1:0EzbDEGsAvOZNbqXopgniY0w0a1phvu5IdUFq8grmqY=
-cloud.google.com/go/longrunning v0.9.0/go.mod h1:pkTz846W7bF4o2SzdWJ40Hu0Re+UoNT6Q5t+igIcb8E=
+cloud.google.com/go/longrunning v0.12.0 h1:wLv2hXvID9zHejLtcPo1B0JBjErnwZCYAPKSTa65xpY=
+cloud.google.com/go/longrunning v0.12.0/go.mod h1:8nqFBPOO1U/XkhWl0I19AMZEphrHi73VNABIpKYaTwM=
 cloud.google.com/go/managedidentities v1.3.0/go.mod h1:UzlW3cBOiPrzucO5qWkNkh0w33KFtBJU281hacNvsdE=
 cloud.google.com/go/managedidentities v1.4.0/go.mod h1:NWSBYbEMgqmbZsLIyKvxrYbtqOsxY1ZrGM+9RgDqInM=
 cloud.google.com/go/managedidentities v1.5.0/go.mod h1:+dWcZ0JlUmpuxpIDfyP5pP5y0bLdRwOS4Lp7gMni/LA=