fix(datafusion): reject non-append insert operations

[u70b3]

Which issue does this PR close?

• Closes DataFusion non-append InsertOp is silently committed as append #2711.

What changes are included in this PR?

Reject DataFusion InsertOp::Overwrite and InsertOp::Replace before constructing an Iceberg write plan.

Previously, IcebergTableProvider::insert_into ignored the requested insert operation while the commit executor always used fast_append. Unsupported overwrite or replace requests could therefore be silently executed with append semantics.

This patch keeps the existing append path unchanged and returns DataFusionError::NotImplemented for unsupported insert operations. It preserves the existing _insert_op parameter name to avoid changing the generated public API snapshot.

Are these changes tested?

Yes. Added unit tests verifying that:

• InsertOp::Overwrite returns DataFusionError::NotImplemented.
• InsertOp::Replace returns DataFusionError::NotImplemented.
• Existing append-insert behavior remains unchanged.

Local verification:

• cargo test -p iceberg-datafusion test_catalog_backed_provider_rejects -- --nocapture
• cargo fmt --all -- --check
• git diff --check

[CTTY]

LGTM! Just one minor comment

[CTTY]

How about merging these 2 tests into 1 test_catalog_backed_provider_rejects_non_append_op

[u70b3]

thanks for the comment, already merge two testcase