Make run.kafka.sasl.mechanism optional

[aliok]

Changes

Follow-up to review feedback on #4069 (thanks @gauron99); reproduced on a live
cluster.

• 🐛 Make run.kafka.sasl.mechanism optional. func-go's Kafka runtime
  defaults an empty mechanism to PLAIN, so a SASL/PLAIN broker deployed and
  consumed fine before scale.keda landed. Requiring the field broke that config
  on every deployer (raw, knative, keda), not only keda. Validation now
  constrains only a non-empty value to the mechanisms both sides understand, and
  kedaSASLType maps "" to KEDA's plaintext so the scaler authenticates the
  same way the function container does.

/kind bug

Relates to #4069

Release Note

Fixed a regression where `run.kafka.sasl.mechanism` was required: it is now optional
and defaults to PLAIN, so SASL/PLAIN Kafka functions that omit it deploy again without
edits.

Docs

[knative-prow]

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[Copilot]

Copilot review overview

🟡 Changes recommended

KEDA metadata must emit SASL plaintext configuration when the mechanism is omitted.

Review effort: Lite
Findings: 1

Open (1)

• Empty SASL mechanism does not emit plaintext metadata · New

What changed in this PR

Makes run.kafka.sasl.mechanism optional, defaulting omitted values to SASL/PLAIN.

Changes:

• Relaxes mechanism validation.
• Maps empty mechanisms to KEDA plaintext.
• Updates tests and documentation.

A critical issue remains: empty mechanisms are not emitted in KEDA ScaledObject metadata.

File	Reviewed changes
pkg/​keda/​kafka_scaling.go	Adds empty-mechanism mapping.
pkg/​keda/​kafka_scaling_test.go	Tests the mapping.
pkg/​keda/​deployer_unit_test.go	Removes obsolete required-mechanism coverage.
pkg/​functions/​function.go	Makes mechanism validation conditional.
pkg/​functions/​function_test.go	Tests empty mechanisms.
docs/​reference/​func_yaml.md	Documents the optional field and default.

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[Copilot]

Copilot review overview

🔵 Needs a closer look

Align the shipped KafkaSASL schema with validation so explicit empty mechanisms are accepted consistently.

Review effort: Lite
Findings: None

Resolved since last review (1)

• Empty SASL mechanism does not emit plaintext metadata