[java-spanner-jdbc] Underlying Java spanner client attempting to use TLS despite usePlainText=true

#### Environment details

OS: Linux (Debian 14)
Java version: 17
java-spanner-jdbc version: 2.32.3

#### Steps to reproduce

1. Attempt to connect via JDBC to local Spanner emulator using usePlainText=true
2. Observe it trying to connect via TLS

#### Code example

```kotlin
fun buildJdbcConnectionString(
emulatorHost: String,
project: String,
instance: String,
database: String,
): String {
return "jdbc:cloudspanner://$emulatorHost/projects/$project/instances/$instance/databases/" +
"$database;usePlainText=true;autoConfigEmulator=true"
}
```

```kotlin
val connectionString =
SpannerEmulator.buildJdbcConnectionString(emulatorHost, PROJECT, INSTANCE, databaseId)
DriverManager.getConnection(connectionString).use { connection ->
Liquibase.update(connection, changelogPath)
}
```

#### Stack trace

There's no exception thrown. This stack trace is from my test timing out.

```
app//com.google.auth.mtls.SecureConnectProvider.runCertificateProviderCommand(SecureConnectProvider.java:163)
at app//com.google.auth.mtls.SecureConnectProvider.getKeyStore(SecureConnectProvider.java:142)
at app//com.google.auth.mtls.SecureConnectProvider.getKeyStore(SecureConnectProvider.java:106)
at app//com.google.auth.mtls.SecureConnectProvider.isAvailable(SecureConnectProvider.java:127)
at app//com.google.auth.mtls.DefaultMtlsProviderFactory.create(DefaultMtlsProviderFactory.java:60)
at app//com.google.api.gax.rpc.EndpointContext$Builder.determineEndpoint(EndpointContext.java:297)
at app//com.google.api.gax.rpc.EndpointContext$Builder.build(EndpointContext.java:411)
at app//com.google.api.gax.rpc.StubSettings.buildEndpointContext(StubSettings.java:129)
at app//com.google.api.gax.rpc.StubSettings.(StubSettings.java:110)
at app//com.google.cloud.spanner.v1.stub.SpannerStubSettings.(SpannerStubSettings.java:405)
at app//com.google.cloud.spanner.v1.stub.SpannerStubSettings$Builder.build(SpannerStubSettings.java:823)
at app//com.google.cloud.spanner.spi.v1.GapicSpannerRpc.(GapicSpannerRpc.java:474)
at app//com.google.cloud.spanner.spi.v1.GapicSpannerRpc.(GapicSpannerRpc.java:290)
at app//com.google.cloud.spanner.SpannerOptions$DefaultSpannerRpcFactory.create(SpannerOptions.java:644)
at app//com.google.cloud.spanner.SpannerOptions$DefaultSpannerRpcFactory.create(SpannerOptions.java:639)
at app//com.google.cloud.ServiceOptions.getRpc(ServiceOptions.java:602)
at app//com.google.cloud.spanner.SpannerOptions.getSpannerRpcV1(SpannerOptions.java:2207)
at app//com.google.cloud.spanner.SpannerImpl.(SpannerImpl.java:155)
at app//com.google.cloud.spanner.SpannerOptions$DefaultSpannerFactory.create(SpannerOptions.java:634)
at app//com.google.cloud.spanner.SpannerOptions$DefaultSpannerFactory.create(SpannerOptions.java:629)
at app//com.google.cloud.ServiceOptions.getService(ServiceOptions.java:582)
at app//com.google.cloud.spanner.connection.SpannerPool.createSpanner(SpannerPool.java:425)
at app//com.google.cloud.spanner.connection.SpannerPool.getSpanner(SpannerPool.java:341)
at app//com.google.cloud.spanner.connection.ConnectionImpl.(ConnectionImpl.java:318)
at app//com.google.cloud.spanner.connection.ConnectionOptions.getConnection(ConnectionOptions.java:893)
at app//com.google.cloud.spanner.jdbc.AbstractJdbcConnection.(AbstractJdbcConnection.java:68)
at app//com.google.cloud.spanner.jdbc.JdbcConnection.(JdbcConnection.java:97)
at app//com.google.cloud.spanner.jdbc.JdbcDriver.connect(JdbcDriver.java:232)
at platform/java.sql@17.0.14/java.sql.DriverManager.getConnection(DriverManager.java:681)
at platform/java.sql@17.0.14/java.sql.DriverManager.getConnection(DriverManager.java:252)
```

#### Any additional information below

I observed this after upgrading my client libraries by updating my version of the [Google Cloud Libraries BOM](https://github.com/googleapis/java-cloud-bom).

It appears that the connection goes through *eventually*, but it's slow enough that my tests now time out.

I see the following warnings repeatedly in my logs:

```
Sep 20, 2025 1:25:25 AM com.google.api.gax.rpc.EndpointContext$Builder determineEndpoint
WARNING: DefaultMtlsProviderFactory encountered unexpected IOException: SecureConnect: Cert provider command failed with exit code: 139
```