fix(connectors): log underlying errors instead of discarding them in SDK poll/consume paths

[atharvalade]

Which issue does this PR close?

Closes #3171

Rationale

Silent error swallowing in connector SDK made debugging connector failures nearly impossible — only generic "Failed to poll/consume" messages appeared with zero root-cause context.

What changed?

The source SDK's handle_messages loop discarded the Err variant from source.poll() and postcard::to_allocvec, logging only a static string. The sink SDK had the same pattern across all postcard::from_bytes deserialization calls, try_into_payload, and sink.consume() (which wasn't logged at all).

All let Ok(x) = expr else { ... } patterns are replaced with match to bind and format the underlying error into the existing log lines. No behavioral change — same control flow (continue/return codes) — just makes the root cause visible.

Local Execution

• Passed / passed
• Pre-commit hooks ran / ran (cargo fmt --check, cargo clippy, trailing-whitespace, trailing-newline, license-headers all green)

AI Usage

1. Opus 4.6
2. used to locate the pattern and write the match arms
3. Verified via cargo clippy, cargo fmt --check, and full CI script suite locally
4. Yes, all code can be explained

[codecov]

Codecov Report

❌ Patch coverage is 55.00000% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.33%. Comparing base (611fca0) to head (d8eee3f).

Files with missing lines	Patch %	Lines
core/connectors/sdk/src/sink.rs	50.00%	15 Missing ⚠️
core/connectors/sdk/src/source.rs	70.00%	3 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3200       +/-   ##
=============================================
- Coverage     74.10%   58.33%   -15.77%     
  Complexity      943      943               
=============================================
  Files          1159     1158        -1     
  Lines        102033    91926    -10107     
  Branches      79083    68976    -10107     
=============================================
- Hits          75607    53623    -21984     
- Misses        23765    35723    +11958     
+ Partials       2661     2580       -81     

Components	Coverage Δ
Rust Core	54.21% <55.00%> (-21.12%)	⬇️
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/sdk/src/source.rs	81.94% <70.00%> (+16.23%)	⬆️
core/connectors/sdk/src/sink.rs	75.75% <50.00%> (-1.07%)	⬇️

... and 245 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.