feat: SQS System Attributes and Custom Attributes Propagation

[cosmic-chichu]

Summary

Enables propagation of SQS attributes through Numaflow pipelines, supporting FIFO queue semantics (MessageGroupId, MessageDeduplicationId) and user-defined message attributes.

Changes

• SQS Source: Attribute extraction
  Renamed SqsMessage fields to system_attributes (SQS system attrs like SentTimestamp, MessageGroupId) and custom_attributes (user-defined message attributes). System attributes become message headers; custom attributes are stored in metadata.user_metadata["sqs"].

• SQS Sink: FIFO queue support
  Added headers field to SqsSinkMessage. The sink reads MessageGroupId, MessageDeduplicationId, and DelaySeconds from headers and applies them to the SQS SendMessageBatch request.

• numaflow-core integration
  Source maps system_attributes → Message.headers and custom_attributes → Message.metadata. Sink merges headers with metadata.user_metadata["sqs"], allowing UDFs/transformers to set or override FIFO attributes.

• Improved error messages
  Changed Error::Sqs to store extracted error details instead of opaque AWS SDK error. Errors now display as "ErrorCode: Error message" instead of "unhandled error (CODE)".

• Documentation
  Added sink documentation (docs/user-guide/sinks/sqs.md), updated source docs with attribute configuration examples, and added SQS example to message headers concept page.

[codecov]

Codecov Report

❌ Patch coverage is 96.94915% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.19%. Comparing base (09d60db) to head (0c00955).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
rust/extns/numaflow-sqs/src/sink.rs	95.00%	3 Missing ⚠️
rust/extns/numaflow-sqs/src/lib.rs	88.88%	2 Missing ⚠️
rust/extns/numaflow-sqs/src/source.rs	90.00%	2 Missing ⚠️
rust/numaflow-core/src/sinker/sink/sqs.rs	98.54%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3095      +/-   ##
==========================================
+ Coverage   80.14%   80.19%   +0.05%     
==========================================
  Files         295      295              
  Lines       66977    67224     +247     
==========================================
+ Hits        53679    53912     +233     
- Misses      12748    12762      +14     
  Partials      550      550              

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[vigith]

why change this?

[cosmic-chichu]

We use Sqs(String) for simpler error propagation since we don't branch on specific AWS error types—we just log and propagate them to the user. This is especially important with metadata propagation, where SQS errors can surface at runtime (not just during initialization), so clear, user-readable error messages are critical.