Add FAIL API to pynumaflow-lite SDK (trfm/map)

[vaibhavtiwari33]

Summary

Add the FAIL API to the pynumaflow-lite SDK so that user-defined functions can explicitly mark a message as failed. A failed message is retried according to the configured retryStrategy, matching the behavior recently introduced in Numaflow's transformer/map retry mechanism.

Background

Numaflow added a retry mechanism for transformer and map UDFs in numaproj/numaflow#3570. As part of that work, the ability to fail a message from within a UDF was exposed across SDKs:

• pynumaflow (full SDK) — feat: introduce API to fail a message in udf/transformer #370
• numaflow-rs — feat: introduce API to fail a message in udf/transformer numaflow-rs#185

The full pynumaflow SDK already provides this API. pynumaflow-lite does not yet expose it, so users on the lite SDK cannot fail/retry a message.

Goal

Provide the FAIL API in pynumaflow-lite for the UDF types that support it in the full SDK, so a handler can return a "failed" message and have it retried per retryStrategy.

Target UDF surfaces (mirroring pynumaflow#370):

• Map
• Batch Map
• Map Stream
• Source Transformer

Acceptance Criteria

• A UDF handler in pynumaflow-lite can mark a message as failed for the map, batch map, map stream, and source transformer types.
• The failure signal is propagated correctly to the Numaflow runtime over the existing gRPC contract (consistent with the wire behavior used by pynumaflow and numaflow-rs).
• Unit tests cover the new API (message-level and, where applicable, server/handler-level).
• Docs / examples updated as needed.

References

• Retry mechanism (core): feat: introduce retry config for transformer and map numaflow#3570
• FAIL API in pynumaflow (full SDK): feat: introduce API to fail a message in udf/transformer #370
• FAIL API in numaflow-rs: feat: introduce API to fail a message in udf/transformer numaflow-rs#185