feat: show MERGE INTO details in EXPLAIN

[wirybeaver]

Which issue does this PR close?

Follow-up to the MERGE INTO review in #22988 and split from #24195. No separate issue.

Rationale for this change

Logical-plan output for MERGE INTO currently shows only op=[MergeInto]. It omits the ON condition and WHEN actions, which makes EXPLAIN less useful for understanding or debugging a merge plan.

What changes are included in this PR?

• Adds display implementations for MERGE operations, clause kinds, clauses, and actions.
• Includes the ON condition and ordered WHEN ... THEN ... clauses in indented logical-plan output.
• Adds the same information to PostgreSQL JSON output and identifies DML nodes as Dml rather than Projection.
• Adds text and JSON assertions covering aliases, quoted columns, predicates, updates, deletes, and inserts.

This PR is independent of #24195 and targets main directly. The two PRs only modify separate expectations in the shared merge_into.slt test file.

What is the testing strategy for this PR?

• cargo fmt --all
• cargo check -p datafusion-expr -p datafusion-sql
• cargo test -p datafusion-expr write_op_merge_into_name_and_display -- --nocapture
• cargo test -p datafusion-sql plan_merge_into_preserves_target_qualifier_and_quoted_columns -- --nocapture
• cargo test --profile ci --test sqllogictests -- merge_into.slt --nocapture
• cargo clippy --all-targets --all-features -- -D warnings
• Full extended workspace test command from the contributor guide

Are there any user-facing changes?

Yes. EXPLAIN text and PostgreSQL JSON output for MERGE INTO now include the merge condition and clauses. There are no public API signature changes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 76.38889% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.66%. Comparing base (c3ef346) to head (4d15378).

Files with missing lines	Patch %	Lines
datafusion/expr/src/logical_plan/dml.rs	73.58%	2 Missing and 12 partials ⚠️
datafusion/expr/src/logical_plan/plan.rs	60.00%	0 Missing and 2 partials ⚠️
datafusion/expr/src/logical_plan/display.rs	92.85%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #26038   +/-   ##
=======================================
  Coverage   82.66%   82.66%           
=======================================
  Files        1147     1147           
  Lines      446357   446426   +69     
  Branches   446357   446426   +69     
=======================================
+ Hits       368971   369041   +70     
+ Misses      54997    54982   -15     
- Partials    22389    22403   +14     

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