Support `system.tables` queries when scoped to a database

### Problem

The `system.tables` table can only be queried on the `_internal` database, e.g.,
```
influxdb3 query --database _internal "select * from system.tables where database_name = ''"
```
but not,
```
influxdb3 query --database "select * from system.tables"
```

### Proposed solution

- Allow `system.tables` to be queried when scoped to a database, as in the above example.
- Only tables from within the scoped database should be returned in the query results.

### Additional context

You can see where the `tables` system table is currently being added to the `SystemTablesProvider` here:
https://github.com/influxdata/influxdb/blob/9fe5b77c382ab9fe5228559689032e907fa5f35f/influxdb3_system_tables/src/lib.rs#L205-L210
But that is only when the query is against the `_internal` database.

Start in influxdb3_system_tables/src/lib.rs around the SystemTablesProvider setup referenced in the issue, then trace how queries scoped to a database select system tables. Update the behavior so system.tables is available for a scoped database and returns only that database's tables; add or run the relevant query coverage if the surrounding code identifies it.

Written by the indexing model from the issue text.