Fix Postgres decimal formatter clippy

[osipovartem]

Which issue does this PR close?

• ClosesFull workspace clippy fails in PostgreSQL decimal formatting #24974.

Rationale for this change

The required cargo clippy --all-targets --all-features -- -D warnings command fails because the PostgreSQL result formatter takes a BigDecimal by value but only reads it.

What changes are included in this PR?

• Accept &BigDecimal in decimal_to_str.
• Borrow the parsed PostgreSQL decimal at the call site.
• Add a direct formatting regression test under the postgres feature.

What is the testing strategy for this PR?

• cargo test -p datafusion-sqllogictest --lib --features postgres engines::conversion
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check

Are there any user-facing changes?

No. The formatted PostgreSQL decimal output is unchanged.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.67%. Comparing base (262936e) to head (d12d695).

Additional details and impacted files

@@ Coverage Diff @@## main #24975 +/- ##
==========================================
- Coverage 81.67% 81.67% -0.01% 
==========================================
Files 1126 1126 Lines 414842 414842 Branches 414842 414842 ==========================================
- Hits 338841 338832 -9 - Misses 56070 56077 +7 - Partials 19931 19933 +2 

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[neilconway]

Thanks @osipovartem