Support INSERT OVERWRITE for memory tables

[osipovartem]

Which issue does this PR close?

• Supports the Rustice Snowflake compatibility case for INSERT OVERWRITE on development memory tables.

Rationale for this change

MemTable currently rejects InsertOp::Overwrite, so valid INSERT OVERWRITE statements fail during physical planning. This prevents DataFusion-backed local environments from matching Snowflake DML behavior even though the memory sink already buffers the complete input before modifying the table.

What changes are included in this PR?

• Add an opt-in overwrite mode to MemSink while preserving append as the default.
• Allow MemTable to plan InsertOp::Overwrite; InsertOp::Replace remains unsupported.
• Replace each target partition only after the input stream completes successfully, so an input error leaves existing batches untouched.
• Add integration coverage for replacing existing rows and clearing a table with an empty overwrite input.

Are these changes tested?

• cargo +1.97.0 fmt --all -- --check
• cargo +1.97.0 test -p datafusion datasource::memory_test --lib (15 passed)
• cargo +1.97.0 clippy --all-targets --all-features -- -D warnings
• Extended workspace suite: all memory-table tests passed. One unrelated existing memory-limit runner failed because the process baseline RSS (158.8 MB) already exceeded its fixed 152.6 MB limit.

Are there any user-facing changes?

Yes. DataFusion memory tables now support INSERT OVERWRITE; append behavior and existing MemSink::try_new callers are unchanged.