Validate implicit if conditions in action.yml files (#317)

## Problem

In workflow YAML files, writing `if: foo == bar` shows an error because `foo` and `bar` are not valid contexts. However, the same invalid expression in an action.yml file showed no error.

## Solution

Add expression validation for implicit `if` conditions in action.yml files, matching the behavior of workflow YAML validation.

## What's new

1. **Pre-if/post-if validation** (node and docker actions)
   - `pre-if: foo == bar` now shows error for unknown context
   - `post-if: unknownFunc()` now shows error for unknown function

2. **Composite step `if` validation** (fix)
   - Errors from `convertToIfCondition` were being lost due to call ordering
   - Now captured correctly by calling conversion before retrieving errors

## Why the refactor?

The diff includes consolidating multiple validation loops into a single `validateAllTokens()` traversal. This matches the pattern used in workflow YAML validation (`additionalValidations`), making the code consistent between the two validation paths.

workflow-parser/src/model/converter/if-condition.ts

@@ -136,3 +136,32 @@ function walkTreeToFindStatusFunctionCalls(tree: Expr | undefined): boolean {
 
   return false;
 }
+
+/**
+ * Validates a pre-if or post-if condition string.
+ * Unlike step if conditions, pre-if and post-if are evaluated as-is by the runner
+ * (they default to always() only when the field is missing entirely).
+ * This function validates the expression and reports errors through the context.
+ *
+ * @param context The template context for error reporting
+ * @param token The token containing the condition
+ * @param condition The condition string to validate
+ * @returns The validated condition string, or undefined on error
+ */
+export function validateRunsIfCondition(
+  context: TemplateContext,
+  token: TemplateToken,
+  condition: string
+): string | undefined {
+  const allowedContext = token.definitionInfo?.allowedContext || [];
+
+  // Validate the expression directly - no wrapping needed for pre-if/post-if
+  try {
+    ExpressionToken.validateExpression(condition, allowedContext);
+  } catch (err) {
+    context.error(token, err as Error);
+    return undefined;
+  }
+
+  return condition;
+}