build(deps): bump cloud.google.com/go/spanner from 1.87.0 to 1.91.0

Update cloud.google.com/go/spanner from 1.87.0 to 1.91.0.

Summary:
- Bumps the direct Spanner client dependency to v1.91.0.
- Pulls in the matching Google API, gRPC, OpenTelemetry, genproto, and related transitive dependency updates selected by Go module resolution.
- Includes upstream Spanner fixes and additions around multiplexed sessions, location-aware routing, Direct Access configuration, and observability.

Validation:
- PR diff is limited to go.mod and go.sum.
- CI lint, test, and readme-sync passed.
- Unresolved review threads: 0.

Author: dependabot[bot]

go.mod

@@ -4,8 +4,8 @@ go 1.25.0
 
 require (
 	cloud.google.com/go v0.123.0
-	cloud.google.com/go/longrunning v0.7.0
-	cloud.google.com/go/spanner v1.87.0
+	cloud.google.com/go/longrunning v0.9.0
+	cloud.google.com/go/spanner v1.91.0
 	cloud.google.com/go/storage v1.56.1
 	github.com/MakeNowJust/heredoc/v2 v2.0.1
 	github.com/alecthomas/kong v1.15.0
@@ -51,23 +51,23 @@ require (
 	go.uber.org/zap v1.27.0
 	golang.org/x/term v0.41.0
 	golang.org/x/time v0.15.0
-	google.golang.org/api v0.265.0
+	google.golang.org/api v0.274.0
 	google.golang.org/genai v1.47.0
-	google.golang.org/grpc v1.79.3
+	google.golang.org/grpc v1.80.0
 	google.golang.org/protobuf v1.36.11
 )
 
 require (
 	cel.dev/expr v0.25.1 // indirect
-	cloud.google.com/go/auth v0.18.1 // indirect
+	cloud.google.com/go/auth v0.18.2 // indirect
 	cloud.google.com/go/auth/oauth2adapt v0.2.8 // indirect
 	cloud.google.com/go/compute/metadata v0.9.0 // indirect
-	cloud.google.com/go/iam v1.5.3 // indirect
-	cloud.google.com/go/monitoring v1.24.3 // indirect
+	cloud.google.com/go/iam v1.7.0 // indirect
+	cloud.google.com/go/monitoring v1.25.0 // indirect
 	dario.cat/mergo v1.0.2 // indirect
 	github.com/Azure/go-ansiterm v0.0.0-20250102033503-faa5f7b0171c // indirect
 	github.com/GoogleCloudPlatform/grpc-gcp-go/grpcgcp v1.6.0 // indirect
-	github.com/GoogleCloudPlatform/opentelemetry-operations-go/detectors/gcp v1.30.0 // indirect
+	github.com/GoogleCloudPlatform/opentelemetry-operations-go/detectors/gcp v1.31.0 // indirect
 	github.com/GoogleCloudPlatform/opentelemetry-operations-go/exporter/metric v0.53.0 // indirect
 	github.com/GoogleCloudPlatform/opentelemetry-operations-go/internal/resourcemapping v0.53.0 // indirect
 	github.com/Masterminds/semver/v3 v3.4.0 // indirect
@@ -113,7 +113,7 @@ require (
 	github.com/google/s2a-go v0.1.9 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/googleapis/enterprise-certificate-proxy v0.3.14 // indirect
-	github.com/googleapis/gax-go/v2 v2.16.0 // indirect
+	github.com/googleapis/gax-go/v2 v2.21.0 // indirect
 	github.com/gorilla/websocket v1.5.3 // indirect
 	github.com/hailocab/go-hostpool v0.0.0-20160125115350-e80d13ce29ed // indirect
 	github.com/hashicorp/golang-lru v1.0.2 // indirect
@@ -169,11 +169,11 @@ require (
 	go.opentelemetry.io/contrib/detectors/gcp v1.39.0 // indirect
 	go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc v0.62.0 // indirect
 	go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp v0.62.0 // indirect
-	go.opentelemetry.io/otel v1.42.0 // indirect
-	go.opentelemetry.io/otel/metric v1.42.0 // indirect
-	go.opentelemetry.io/otel/sdk v1.42.0 // indirect
-	go.opentelemetry.io/otel/sdk/metric v1.42.0 // indirect
-	go.opentelemetry.io/otel/trace v1.42.0 // indirect
+	go.opentelemetry.io/otel v1.43.0 // indirect
+	go.opentelemetry.io/otel/metric v1.43.0 // indirect
+	go.opentelemetry.io/otel/sdk v1.43.0 // indirect
+	go.opentelemetry.io/otel/sdk/metric v1.43.0 // indirect
+	go.opentelemetry.io/otel/trace v1.43.0 // indirect
 	go.uber.org/multierr v1.11.0 // indirect
 	golang.org/x/crypto v0.49.0 // indirect
 	golang.org/x/exp v0.0.0-20250819193227-8b4c13bb791b // indirect
@@ -184,9 +184,9 @@ require (
 	golang.org/x/sys v0.42.0 // indirect
 	golang.org/x/text v0.35.0 // indirect
 	golang.org/x/tools v0.42.0 // indirect
-	google.golang.org/genproto v0.0.0-20251202230838-ff82c1b0f217 // indirect
-	google.golang.org/genproto/googleapis/api v0.0.0-20260316180232-0b37fe3546d5 // indirect
-	google.golang.org/genproto/googleapis/rpc v0.0.0-20260319201613-d00831a3d3e7 // indirect
+	google.golang.org/genproto v0.0.0-20260319201613-d00831a3d3e7 // indirect
+	google.golang.org/genproto/googleapis/api v0.0.0-20260401024825-9d38bb4040a9 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20260401024825-9d38bb4040a9 // indirect
 	gopkg.in/inf.v0 v0.9.1 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )