DHIS2-21730: Emit effects for assignments to program rule variables backed by a field

Summary

ASSIGN actions targeting a program rule variable (content = #{variableName} or A{variableName}) previously only mutated the in-memory value map, so the assigned value was never propagated back to the data element or tracked entity attribute the variable is backed by. This PR makes the engine emit a RuleEffect targeting the backing field for such assignments, and hardens validation and error reporting around ASSIGN targets.

Changes

ASSIGN target resolution (RuleAction.assignTarget() / AssignTarget)

• New sealed class AssignTarget with three cases:
  • Field — assign to the data element/attribute named by the action's field; emits an effect for the caller to apply.
  • Variable — assign to a program rule variable referenced in content; updates the evaluation state so subsequent rules see the new value. Both #{variableName} and A{variableName} notations are accepted.
  • Invalid — the action defines neither field nor content, or content is not a variable reference; carries a human-readable reason.
• RuleAction gains ASSIGN/ERROR type constants and an internal withAssignBacking(field, attributeType) helper.

Effect emission for variable assignments (RuleConditionEvaluator)

• When the assigned variable is backed by a data element or tracked entity attribute, the engine now also emits a RuleEffect carrying a copy of the action with the backing field and attributeType injected, making the effect self-contained for consumers.
• Each assignment is emitted exactly once across evaluation passes:
  • data-element backed variables → event pass;
  • attribute backed variables → enrollment pass when the execution includes one (hasEnrollmentPass), otherwise the event pass emits them (single-event evaluation, executions without an enrollment, rules scoped to a program stage), so assignments are never silently dropped.
• Calculated values have no backing field and emit no effect.
• Assigning to a name that is not a defined program rule variable still updates the value map but logs a warning (possible typo) and emits no effect.

Validation & error reporting

• New internal RuleConfigurationException: malformed ASSIGN targets (AssignTarget.Invalid) are reported as error effects, like expression errors, instead of surfacing as unexpected exceptions.
• Error-message construction in addRuleErrorResult deduplicated.
• Removed the public unwrapVariableName utility (superseded by assignTarget()); currentDate() is now internal.

Logger

• Logger gains a warning level, wired up on JVM (Level.WARNING), JS (console.warn) and native.