Fix Postgres decimal formatter clippy

[osipovartem]

Which issue does this PR close?

• Closes Full 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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