[SPARK-48310][PYTHON][CONNECT] Cached properties must return copies

[grundprinzip]

What changes were proposed in this pull request?

When a consumer modifies the result values of a cached property it will modify the value of the cached property.

Before:

df_columns = df.columns
for col in ['id', 'name']:
  df_columns.remove(col)
assert len(df_columns) == df.columns

But this is wrong and this patch fixes it to

df_columns = df.columns
for col in ['id', 'name']:
  df_columns.remove(col)
assert len(df_columns) != df.columns

Why are the changes needed?

Correctness of the API

Does this PR introduce any user-facing change?

No, this makes the code consistent with Spark classic.

How was this patch tested?

UT

Was this patch authored or co-authored using generative AI tooling?

No

[hvanhovell]

LGTM

[HyukjinKwon]

Merged to master.