fix(connectors): unify extract_column_value into single free function

[atharvalade]

Which issue does this PR close?

Closes #3172

Rationale

Bug fixes or new type support added to one copy of extract_column_value silently won't apply to the other, causing divergence between sequential and parallel code paths.

What changed?

The Postgres source connector had extract_column_value as a method on PostgresSource (&self), making it impossible to call from static/parallel contexts without duplicating the function. Any future parallel/chunked path would need its own copy.

Converted it to a single free function callable from any context. Added missing DATE (via chrono::NaiveDate) and BPCHAR type handling that the parallel copy on the benchmark branch had but this version lacked.

Local Execution

• Passed
• Pre-commit hooks ran

AI Usage

1. Opus 4.6
2. exploration and implementation guidance
3. Verified via cargo check, clippy -D warnings, fmt --check, all 19 unit tests passing, CI lint scripts passing
4. Yes, all code can be explained

[codecov]

Codecov Report

❌ Patch coverage is 0% with 108 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.21%. Comparing base (611fca0) to head (08153c5).

Files with missing lines	Patch %	Lines
core/connectors/sources/postgres_source/src/lib.rs	0.00%	108 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3199       +/-   ##
=============================================
- Coverage     74.10%   19.21%   -54.89%     
  Complexity      943      943               
=============================================
  Files          1159     1157        -2     
  Lines        102033    90323    -11710     
  Branches      79083    67391    -11692     
=============================================
- Hits          75607    17356    -58251     
- Misses        23765    72553    +48788     
+ Partials       2661      414     -2247     

Components	Coverage Δ
Rust Core	0.95% <0.00%> (-74.38%)	⬇️
Java SDK	60.14% <ø> (ø)
C# SDK	69.11% <ø> (-0.28%)	⬇️
Python SDK	81.43% <ø> (ø)
Node SDK	91.53% <ø> (ø)
Go SDK	39.43% <ø> (ø)

Files with missing lines	Coverage Δ
core/connectors/sources/postgres_source/src/lib.rs	41.71% <0.00%> (-25.47%)	⬇️

... and 657 files with indirect coverage changes

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[lukaszzborek]

@atharvalade
In issue is Postgres source connector has two nearly identical functions and to remove one. But i don't see second one.
I'm jus asking for clarification, if i don't see something

Also remeber about #3196 where you also add Date