refactor(connectors): extract row-processing logic into reusable process_row method

[atharvalade]

Which issue does this PR close?

Closes #3173

Rationale

Row-processing logic lived inline in poll_tables, making it impossible to reuse across future parallel/chunked polling paths without duplication.

What changed?

The column-iteration, value-extraction, JSON-payload construction, and ProducedMessage assembly were tightly coupled inside poll_tables's inner loop (~80 lines). Any new polling variant (parallel, chunked) would need to copy-paste this block.

Extracted a process_row method that accepts a RowProcessingConfig (table, tracking/pk columns, format flags) and returns a ProcessedRow (message + offset + pk). The poll_tables loop now delegates to this single-responsibility method in ~12 lines.

Local Execution

• Passed
• Pre-commit hooks ran

AI Usage

1. Opus 4.6
2. used for 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 87 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.22%. Comparing base (611fca0) to head (9cc08ad).

Files with missing lines	Patch %	Lines
core/connectors/sources/postgres_source/src/lib.rs	0.00%	87 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3198       +/-   ##
=============================================
- Coverage     74.10%   19.22%   -54.88%     
  Complexity      943      943               
=============================================
  Files          1159     1157        -2     
  Lines        102033    90344    -11689     
  Branches      79083    67394    -11689     
=============================================
- Hits          75607    17371    -58236     
- Misses        23765    72575    +48810     
+ Partials       2661      398     -2263     

Components	Coverage Δ
Rust Core	0.95% <0.00%> (-74.38%)	⬇️
Java SDK	60.14% <ø> (ø)
C# SDK	69.38% <ø> (ø)
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	40.78% <0.00%> (-26.40%)	⬇️

... and 647 files with indirect coverage changes

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