feat(function): expose producer configuration on pulsar_function

[david-streamlio]

Master Issue: #220

Covers the part of #220 that has no upstream dependency. The batching_config block is deliberately out of scope — see below.

Motivation

pulsar_source exposes its producer configuration as top-level attributes (resource_pulsar_source.go): max_pending_messages, max_pending_messages_across_partitions, use_thread_local_producers, batch_builder, compression_type.

pulsar_function exposes none of it. utils.FunctionConfig carries the same ProducerConfig *ProducerConfig field, but marshalFunctionConfig never populated it, so it was always sent nil — a function's output producer could not be configured from Terraform at all, including its compression.

Modifications

• Add the five attributes to pulsar_function, reusing pulsar_source's names and description strings verbatim so the two resources read the same way.
• Populate FunctionConfig.ProducerConfig in marshalFunctionConfig via a new marshalFunctionProducerConfig, following the shape pulsar_source uses.
• Read them back in unmarshalFunctionProducerConfig, surfacing each field only when the server returned something for it so a function that configures none of them does not gain a diff.
• Regenerate docs/resources/function.md.

marshalFunctionProducerConfig returns nil when none of the attributes are set, so the request for a function that does not configure a producer is byte-identical to before.

Out of scope, deliberately: batching_config. Two independent reasons, either of which alone would block it:

1. PIP-401's batchingConfig is absent from utils.ProducerConfig in the pinned admin client, so there is nothing to serialize into. Tracked as pulsaradmin: ProducerConfig cannot express PIP-401 batchingConfig apache/pulsar-client-go#1528, with a fix open at feat(pulsaradmin): add PIP-401 batchingConfig to ProducerConfig apache/pulsar-client-go#1530.
2. It requires a broker running Pulsar 4.1.0 or later — BatchingConfig.java does not exist on the v4.0.x tags. The acceptance tests here run against 4.0.3 (hack/pulsarimage/Dockerfile), so it could not be acceptance-tested on the current baseline without bumping the image or scoping those tests around it. Worth deciding separately.

Worth knowing about runtime support. compression_type and batch_builder are honoured by the Python function runtime; max_pending_messages is not (the Python runtime reads only compressionType and batchBuilder from the ProducerSpec). The provider should still send all of them — the broker stores them and the Java runtime applies them — but a Python function author should not expect max_pending_messages to take effect today. The Go runtime has the same gap, tracked at apache/pulsar#26391.

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• pulsar/testdata/function/main.tf sets all five attributes, and TestFunction asserts each one round-trips through a real broker.
• Confirmed the assertions are not vacuous: disabling the functionConfig.ProducerConfig = ... assignment and re-running fails five assertions.
• Acceptance tests pass against Pulsar 4.0.3; make test and golangci-lint are clean; go generate ./... reproduces the committed docs.

Documentation

Check the box below.

Need to update docs?

• doc-required

• no-need-doc

• doc

  docs/resources/function.md regenerated with the five new attributes.

[freeznet]

Rebased this branch onto current master (including #219 and #226) and pushed the producer update/read-back fixes in 3447080.

Changes:

• resolve the function schema, test, and fixture conflicts with feat(function): expose input_specs for per-topic consumer config #219;
• write zero, false, and empty producer values back to Terraform state so removed or externally reset settings do not leave stale state;
• send the Function default producer config when the final producer override is removed on update, instead of sending nil and letting Pulsar preserve the old config;
• correct the documented Function compression default to LZ4;
• extend TestFunction to verify the producer settings are cleared in place, plus add a focused zero-value state regression test.

Validation:

• GOTOOLCHAIN=go1.25.14+auto GOWORK=off go test ./...
• GOTOOLCHAIN=go1.25.14+auto GOWORK=off go vet ./...
• go generate ./...
• Pulsar 4.0.3: TF_ACC=1 ... go test ./pulsar -run '^TestFunction$' -count=1 -timeout=20m -v

batching_config intentionally remains outside this PR; it needs a separate, version-scoped change that can be verified across the supported broker and runtime paths.