Tag based functionality not added when using a spanner connection

When using the library with a connection string it adds in two interceptors that are used to add a tag to underling commands among other things:

https://github.com/googleapis/dotnet-spanner-entity-framework/blob/8ef23f3a71130714d6ea40966d54f551c609d232/Google.Cloud.EntityFrameworkCore.Spanner/Extensions/SpannerDbContextOptionsExtensions.cs#L123

However if we use a spanner connection directly these interceptors aren't added in, meaning this functionality doesn't work:

https://github.com/googleapis/dotnet-spanner-entity-framework/blob/8ef23f3a71130714d6ea40966d54f551c609d232/Google.Cloud.EntityFrameworkCore.Spanner/Extensions/SpannerDbContextOptionsExtensions.cs#L80-L81

#### Environment details

- Programming language: C#
- OS:
- Language runtime version: dotnet 10
- Package version: 3.10.0 / 4.0.0

#### Steps to reproduce

1. Use a spanner connection when setting up EF
2. Use `WithRequestTag` extension
3. Check whether spanner tag was applied
4. Making sure to follow these steps will guarantee the quickest resolution possible.