[PR] [fix][functions] Forward source message properties in Go functions [pulsar]

["Asthenia0412 (via GitHub)" <gi...@apache.org>]

Asthenia0412 opened a new pull request, #26728:
URL: https://github.com/apache/pulsar/pull/26728

   ## Motivation
   
   The Go Functions runtime ignores `SinkSpec.forwardSourceMessageProperty`, so output messages silently lose source properties even when forwarding is enabled. This breaks metadata propagation in chained functions and omits the input topic and message ID properties used by the runtime.
   
   ## Modifications
   
   Copy source properties to the output message when forwarding is enabled, then set the runtime-owned input topic and serialized message ID properties so they take precedence over colliding source keys. Add coverage for enabled and disabled forwarding, reserved keys, and preserving the input property map.
   
   Fixes #26408
   
   ## Verification
   
   - `go build ./...`
   - `go test -v $(go list ./... | grep -v examples)`
   - `golangci-lint run -c ./golangci.yml ./pf` (v2.12.2)
   


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