[fix][fn] Allow retainKeyOrdering on Go functions

[david-streamlio]

Master Issue: #26404
Follow-up to #26414, which closed #26405.

Motivation

#26414 taught the Go runtime to honour retainOrdering and retainKeyOrdering: resolveSubscriptionType in pulsar-function-go/pf/instance.go selects Failover for the first and KeyShared for the second, matching python_instance.py.

Half of it cannot be reached. doGolangChecks still refuses retainKeyOrdering outright:

if (functionConfig.getRetainKeyOrdering() != null && functionConfig.getRetainKeyOrdering()) {
    throw new IllegalArgumentException("Retain Key Orderering not yet supported in Go function");
}

validateNonJavaFunction has a single caller, the worker REST API (FunctionsImpl), so cluster submission is exactly what the guard blocks. LocalRunner never calls it, which is why the KeyShared branch is reachable under localrun and nowhere else. As master stands, pulsar-admin functions create --go ... --retain-key-ordering fails at creation, and the code #26414 added for it is dead on the cluster path.

retainOrdering is unaffected — it was never gated — so that half of #26414 works today.

This is the same shape of gap as the Python dead letter queue in #26400, with one difference: convert carries retainKeyOrdering into FunctionDetails unconditionally (L297), so this guard is the only thing in the way.

#26405 was auto-closed when #26414 merged. Its body only described the runtime gap, so the validation half was never in its scope; happy to reopen it instead of tracking this here if that is preferred.

Modifications

Drop the guard from doGolangChecks.

The combinations that would genuinely be contradictory are already rejected in doCommonChecks, for every runtime:

• retainKeyOrdering with EFFECTIVELY_ONCE — "When effectively once processing guarantee is specified, retain Key ordering cannot be set"
• retainKeyOrdering together with retainOrdering — "Only one of retain ordering or retain key ordering can be set"

doGolangChecks refuses EFFECTIVELY_ONCE for Go before either is reached, so nothing needs repeating for Go specifically. The maxMessageRetries guard stays — the Go runtime does not honour retryDetails yet (#26406) — and a test pins it so this change cannot be widened by accident.

The Go client needs no extra configuration for KeyShared: ConsumerOptions leaves KeySharedPolicy nil, toProtoKeySharedMeta(nil) returns nil, and the broker then applies its default auto-split hash range.

Correct the CLI runtime markers. The @Option descriptions in CmdFunctions carry a marker that the docs sync parses into the Support column of the published pulsar-admin CLI reference, and that functions create --help prints verbatim. --retain-ordering and --retain-key-ordering were both #Java, which has been wrong for Python since long before #26414 — python_instance.py has always selected Failover and KeyShared for them. Both now read #Java, Python, Go.

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• Five tests in FunctionConfigUtilsTest: Go accepts retainKeyOrdering and converts to a KEY_SHARED subscription; accepts retainOrdering and converts to FAILOVER; rejects both ordering modes together; rejects retainKeyOrdering with EFFECTIVELY_ONCE; and still refuses message retries.
• FunctionConfigUtilsTest: 42/42 pass.
• Confirmed the tests are not vacuous: restoring the guard fails testGoFunctionAcceptsRetainKeyOrdering and nothing else.

Does this pull request potentially affect one of the following parts:

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints

This is relaxing only. A configuration that was accepted before is still accepted; one input that was rejected — retainKeyOrdering on a Go function — is now accepted, and behaves as the runtime already implements. Java and Python functions are untouched.

Documentation

• doc-required
• doc-not-needed

The FunctionConfig table in docs/functions-cli.md documents retainOrdering and retainKeyOrdering with no runtime qualification. With this change that is accurate for the first time — all three runtimes honour both — so the table needs no edit. The #Java, Python, Go markers above keep the generated CLI reference in step.

[freeznet]

The validation and conversion paths are consistent with the Java and Python function behavior. I left one non-blocking simplification suggestion inline.