Support INSERT OVERWRITE for MemTable

[osipovartem]

Which issue does this PR close?

• Part of [EPIC] Complete DML Support (MERGE, INSERT OVERWRITE, TRUNCATE) #19617.

Rationale for this change

DataFusion parses and plans INSERT OVERWRITE, but MemTable rejects every insert operation except append. Users therefore cannot replace the contents of an in-memory table with the standard overwrite operation.

What changes are included in this PR?

• allow InsertOp::Overwrite in MemTable::insert_into
• let MemSink replace each target partition after the input stream completes successfully
• continue rejecting the distinct InsertOp::Replace operation

What is the testing strategy for this PR?

Two unit tests verify that overwrite replaces existing rows and that an empty overwrite clears the table.

Validated with:

• cargo test -p datafusion --lib test_insert_overwrite -- --nocapture
• cargo clippy -p datafusion -p datafusion-datasource -p datafusion-catalog --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check

Are there any user-facing changes?

Yes. INSERT OVERWRITE now replaces all existing data in a MemTable. There are no breaking public API changes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.12346% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.67%. Comparing base (262936e) to head (6a6f047).

Files with missing lines	Patch %	Lines
datafusion/core/src/datasource/memory_test.rs	89.85%	4 Missing and 3 partials ⚠️
datafusion/catalog/src/memory/table.rs	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #24969   +/-   ##
=======================================
  Coverage   81.67%   81.67%           
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  Files        1126     1126           
  Lines      414842   414919   +77     
  Branches   414842   414919   +77     
=======================================
+ Hits       338841   338905   +64     
- Misses      56070    56079    +9     
- Partials    19931    19935    +4     

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