fix(connectors): Add unwrap_envelope transform and envelope detection to fix source-sink format mismatch

[atharvalade]

Which issue does this PR close?

Closes #3174

Rationale

Sources (e.g. Postgres) wrap row data in a DatabaseRecord envelope while sinks (e.g. Iceberg) expect flat JSON matching the target table schema — no shared contract exists, producing silent null failures.

What changed?

The Postgres source emits {table_name, operation_type, timestamp, data: {...}, old_data} envelopes, but the Iceberg sink's Arrow JSON reader maps these nested structures to top-level fields as null, silently violating non-nullable constraints.

This adds a reusable unwrap_envelope transform to the connector SDK that extracts a nested field (e.g. data) and promotes it as the top-level payload, plus explicit envelope detection in the Iceberg sink that errors with an actionable message instead of failing silently.

Local Execution

• Passed
• Pre-commit hooks ran (fmt, clippy, license-headers, trailing-whitespace, trailing-newline all clean; 119 tests pass across SDK + integration suites)

AI Usage

1. Opus 4.6
2. used for codebase exploration and following existing transform patterns
3. All 8 new unit tests pass locally, clippy/fmt clean, existing 111 tests unaffected
4. Yes, all code can be explained

[codecov]

Codecov Report

❌ Patch coverage is 91.27907% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.37%. Comparing base (611fca0) to head (8defa26).

Files with missing lines	Patch %	Lines
...nectors/sdk/src/transforms/json/unwrap_envelope.rs	96.52%	5 Missing ⚠️
...re/connectors/sinks/iceberg_sink/src/router/mod.rs	60.00%	3 Missing and 1 partial ⚠️
core/connectors/sdk/src/transforms/mod.rs	0.00%	3 Missing ⚠️
...e/connectors/sdk/src/transforms/unwrap_envelope.rs	80.00%	3 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3197       +/-   ##
=============================================
- Coverage     74.10%   58.37%   -15.73%     
  Complexity      943      943               
=============================================
  Files          1159     1160        +1     
  Lines        102033    92080     -9953     
  Branches      79083    69130     -9953     
=============================================
- Hits          75607    53755    -21852     
- Misses        23765    35741    +11976     
+ Partials       2661     2584       -77     

Components	Coverage Δ
Rust Core	54.28% <91.27%> (-21.05%)	⬇️
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/transforms/json/mod.rs	53.84% <ø> (ø)
core/connectors/sdk/src/transforms/mod.rs	29.62% <0.00%> (-3.71%)	⬇️
...e/connectors/sdk/src/transforms/unwrap_envelope.rs	80.00% <80.00%> (ø)
...re/connectors/sinks/iceberg_sink/src/router/mod.rs	40.71% <60.00%> (+1.48%)	⬆️
...nectors/sdk/src/transforms/json/unwrap_envelope.rs	96.52% <96.52%> (ø)

... and 248 files with indirect coverage changes

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