Undefined JSONPath expressions produce nil silently — hard to debug #12

Problem

When a JSONPath expression in a pipeline resolves to nothing (undefined field, typo), GetJSONPathRaw returns (nil, nil) by design. This is needed for @definedOr to work. But in @project context, nil values flow into the output object silently:

• Critical fields (metadata.name/namespace): K8s rejects the object with a confusing error like "metadata/namespace must be a string (current value nil)" — no indication which expression failed
• Non-critical fields (labels, config values, resources): silently null in the output, no error anywhere

Example

# Typo: $.MatrixRTCStack.metadata.namespace in a single-source pipeline
# (should be $.metadata.namespace — single-source doesn't use Kind prefix)
metadata:
  namespace: "$.MatrixRTCStack.metadata.namespace"

Error at runtime: invalid object: metadata/namespace must be a string (current value nil)

The actual cause (wrong JSONPath convention) is not visible in any log or event.

Root Cause

pkg/expression/jsonpath.go:108-111 — error return deliberately commented out, replaced with return nil, nil. This is correct for @definedOr (line 309 checks v == nil).

Suggestion

Add a DEBUG-level log when a JSONPath resolves to nil outside of @definedOr context, e.g.:

DEBUG  expression  JSONPath resolved to nil  {"expression": "$.MatrixRTCStack.metadata.namespace", "context": "@project"}

This preserves backward compatibility and @definedOr behavior while making typos visible in logs.