Core: Schema for a branch should return table schema

[nastra]

When retrieving the schema for branch we should always return the table schema instead of the snapshot schema. This is because the table schema is the schema that will be used when the branch will be created. We should only return the schema of the snapshot when we have a tag.
Below is an example that shows the weird schema behavior when describing a table.

-- create a table with a single column and insert a value
spark-sql (default)> create table t (s string);
spark-sql (default)> insert into t values ('foo');
-- create a branch, the schema is the same as the original table
spark-sql (default)> alter table t create branch b1;

spark-sql (default)> describe default.t;
s                       string                      --> this schema comes from top-level table metadata
spark-sql (default)> describe default.t.branch_b1;
s                       string                      --> this is the same schema, but comes from the snapshot with the record ('foo')
-- alter the table schema and now the definitions diverge
spark-sql (default)> alter table t add column i int;

spark-sql (default)> describe default.t;
s                       string
i                       int
spark-sql (default)> describe default.t.branch_b1;
s                       string

-- insert into the branch and the schema changes back to the top-level schmea
spark-sql (default)> insert into default.t.branch_b1 values ('bar');

spark-sql (default)> describe default.t.branch_b1;
s                       string
i                       int

[rdblue]

Can you use SimpleRecord like the rest of the tests do instead?

[nastra]

unfortunately that doesn't work, because SimpleRecord expects the data field to be populated. The particular error is [UNRESOLVED_COLUMN.WITH_SUGGESTION] A column or function parameter with name data cannot be resolved. Did you mean one of the following? [id].

[nastra]

I'm not sure whether we actually want to fix this as part of this PR or a separate PR, but in the Iceberg sync we briefly talked about making sure that SELECT * from ns.table.branch_x VERSION AS OF ... shouldn't be supported and should throw an error, which is what this check is doing

[rdblue]

Probably a separate PR.

[nastra]

I've applied this and moved this to #9219

[rdblue]

This wasn't introduced by this commit, but branch should be final right?

[nastra]

it is effectively final as it's only set once. However, it's not marked as final due to the way the different constructors in SparkTable are called

[rdblue]

I don't think this is an appropriate place for the write test. It should be a new test case because this case tests the schema that is used when reading.

In addition, the test case should test writing when the current snapshot for a branch has a different schema than the table schema. With the column added back, the schemas are the same.

[nastra]

I've moved this to a separate test and also used a different schema

[nastra]

Thanks for reviewing this @rdblue, I've applied your feedback and also moved the VERSION AS OF handling to #9219