Generics not correctly deserialized for Spanner JSON columns

**Describe the bug**
Deserializing generic types with spring cloud gcp spanner does not work.
If I have classes like this:
```kotlin
@Table(name = "MyEntity")
class MyEntity (
val id: String,
@Column(name = "jsonField", spannerType = TypeCode.JSON)
val myGenericJsonField: GenericType,
)

data class GenericType (
val genericField: T
)

data class SimpleType(
val test: String
)
```

with the following schema:
```sql
CREATE TABLE IF NOT EXISTS MyEntity(
id STRING(36) PRIMARY KEY,
jsonField JSON
)
```

After writing to spanner and reading the entity back it will not restore the generic type correctly. So instead of `genericField` being `SimpleType`, it fills it with a `com.google.gson.internal.LinkedTreeMap`.

The problem seems to be that in `SpannerPersistenPropertyImpl.getColumnInnerType` not the whole `TypeInformation` is preserved, so later there is no way to give gson a `TypeToken` to actually reconstruct the correct type.

**Sample**
There is a reproduction repo here: https://github.com/hesch/spring-cloud-gcp-generic-repro

With a test that fails here: https://github.com/hesch/spring-cloud-gcp-generic-repro/blob/main/src/test/kotlin/org/example/GenericTest.kt

Start with SpannerPersistenPropertyImpl.getColumnInnerType and the failing GenericTest.kt in the linked reproduction repository. Trace the Spanner JSON deserialization path and use that test to verify that a round trip preserves GenericType instead of producing a LinkedTreeMap.

Written by the indexing model from the issue text.