feat: add basic MemTable MERGE INTO support

[wirybeaver]

Which issue does this PR close?

Follow-up to the MERGE INTO review in #22988. No separate issue.

Rationale for this change

MERGE INTO could be planned, but the built-in in-memory table provider could not execute it, and EXPLAIN only showed op=[MergeInto] without the ON condition or WHEN actions. This made the new SQL surface hard to exercise end-to-end with the default provider.

What changes are included in this PR?

• Adds display helpers for MERGE INTO operations, clauses, and actions, and includes ON / WHEN ... THEN ... details in text and JSON logical plan output.
• Implements basic MemTable MERGE INTO execution for matched update/delete, not-matched insert, and not-matched-by-source update/delete.
• Handles first matching clause semantics, NULL predicates as false, duplicate source matches for a target row as an error before mutation, insert column subsets with defaults or typed NULLs, expression casts to target column types, affected-row counts, and sort-order reset.
• Documents MERGE INTO syntax, provider caveats, and currently unsupported planner syntax.
• Extends sqllogictests and Rust tests for planning, display, execution, NULL/no-op predicates, defaults, and duplicate-match errors.

Are these changes tested?

Yes:

• cargo fmt --all
• cargo test -p datafusion-expr
• cargo test -p datafusion-catalog
• cargo test -p datafusion --test core_integration merge_into -- --nocapture
• cargo test -p datafusion-sql plan_merge_into_canonicalizes_qualifiers_and_preserves_quoted_columns -- --nocapture
• cargo test --profile=ci --test sqllogictests -- merge_into.slt --nocapture
• ./ci/scripts/doc_prettier_check.sh --write --allow-dirty
• cargo clippy --all-targets --all-features -- -D warnings

Note: the plan mentioned cargo test -p datafusion --test sql, but this repository does not currently have a sql test target; the relevant MERGE SQL tests live under core_integration.

Are there any user-facing changes?

Yes. MemTable now supports basic MERGE INTO, and EXPLAIN output for MERGE INTO includes the merge condition and clauses. There are no public API signature changes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 79.35657% with 77 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.04%. Comparing base (eec8b94) to head (4e823ba).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/catalog/src/memory/table.rs	81.06%	32 Missing and 25 partials ⚠️
datafusion/expr/src/logical_plan/dml.rs	69.09%	3 Missing and 14 partials ⚠️
datafusion/expr/src/logical_plan/plan.rs	60.00%	0 Missing and 2 partials ⚠️
datafusion/expr/src/logical_plan/display.rs	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24195      +/-   ##
==========================================
- Coverage   81.05%   81.04%   -0.01%     
==========================================
  Files        1106     1106              
  Lines      382287   382658     +371     
  Branches   382287   382658     +371     
==========================================
+ Hits       309851   310136     +285     
- Misses      54121    54162      +41     
- Partials    18315    18360      +45     

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