Improve YAML parsing robustness and handle null Avro types

[jogrogan]

Problem

Improve YAML parsing robustness and handle null Avro types

The operator processes Kubernetes resource YAML that can be arbitrarily malformed — truncated, invalid mappings, duplicate document markers, or Java type tags. Several code paths were either throwing these exceptions to callers when they should have been handling them gracefully, or logging caught exceptions at ERROR level when WARN is more appropriate (since the code recovers successfully).

Additionally, AvroConverter did not handle Avro schemas with null types, causing NullPointerException during SQL planning for schemas with optional fields.

Changes

K8sPipelineElementStatusEstimator

• Downgraded YAML parse exception log level from ERROR to WARN. The exceptions are caught and handled correctly by returning "not ready" status — ERROR was a false signal.

Operator

• Added null-safety guards in isReady() and isFailed() for resources with no namespace or no metadata. Both methods now return false rather than throwing IllegalArgumentException or NullPointerException.

SubscriptionReconciler

• Added null metadata guard in fetchAttributes() to return an empty map rather than throwing.
• Added isSqlValidationError() helper to downgrade SQL validation errors (caused by bad user-supplied SQL) from ERROR to WARN.

AvroConverter

• Added null guard at the entry of rel() for null schema parameters.
• Added explicit case NULL: handler returning SqlTypeName.NULL for null-typed Avro fields.
• Nullable union fields (["null", "T"]) were already handled; the null schema case was the gap.

Tests
New tests covering each scenario:

• K8sPipelineElementStatusEstimatorTest: malformed YAML (IndexOutOfBounds), ScannerException, ParserException, ConstructorException, null metadata
• OperatorTest: ScannerException and ParserException in isReady()/isFailed(), null namespace, null metadata
• SubscriptionReconcilerFetchAttributesTest: ClassCastException, ScannerException, null metadata in fetchAttributes()
• AvroConverterTest: nullable union fields, null-typed fields, null schema parameter