Add missing validation for action.yml (parity with workflow files) (#311)

* Add missing validation for action.yml (parity with workflow files)

- Add uses format validation for composite action steps
  - Validates owner/repo@ref format
  - Supports docker:// and ./ local references
  - Warns about shortened SHA refs (security concern)
  - Detects reusable workflow references in wrong context

- Add if literal text detection for composite action steps
  - Detects literal text outside ${{ }} that makes conditions always truthy
  - Works for both plain string and mixed expression formats
  - Uses shared hasFormatWithLiteralText() utility

- Add pre-if/post-if validation for node and docker actions
  - Errors on explicit ${{ }} syntax (runner only supports implicit expressions)
  - Literal text detection for implicit expressions
  - New runs-if schema type with proper context (runner, github, job, env, inputs, status functions)
  - Validates only in strict schema used by language services

- Add format() function validation for all expressions
  - Validates format string syntax in all expression contexts
  - Checks argument count matches placeholders

- Fix env and matrix context providers to return complete=false
  - Prevents false positive 'unknown context' errors
  - Matches behavior of other dynamic contexts (secrets, vars, etc.)

- Refactor validation utilities into utils/validate-uses.ts and utils/validate-if.ts
  - Shared between workflow and action validation
  - Consistent error messages and codes

* Add strategy and matrix contexts to runs-if definition

Based on runner source code analysis (actions/runner):
- ExecutionContext.InitializeJob() populates ExpressionValues from message.ContextData
- strategy and matrix are part of message.ContextData, available before any steps run
- StepsRunner evaluates all steps (pre, main, post) using the same code path

Did NOT add:
- steps: empty at pre-if time (no steps completed yet)
- hashFiles: workspace files don't exist at pre-step time

languageservice/src/context-providers/default.ts

@@ -198,9 +198,13 @@ function getDefaultActionContext(
     case "runner":
       return getRunnerContext();
 
-    case "env":
-      // Actions can access env but we don't have runtime values
-      return new DescriptionDictionary();
+    case "env": {
+      // Actions can access env but we don't know what env vars the calling workflow defines
+      // Mark as incomplete to avoid false positive "Context access might be invalid" warnings
+      const envContext = new DescriptionDictionary();
+      envContext.complete = false;
+      return envContext;
+    }
 
     case "job": {
       // https://docs.github.com/en/actions/learn-github-actions/contexts#job-context
@@ -218,9 +222,13 @@ function getDefaultActionContext(
     case "strategy":
       return getStrategyContext();
 
-    case "matrix":
-      // Actions can access matrix context at runtime
-      return new DescriptionDictionary();
+    case "matrix": {
+      // Actions can access matrix context at runtime but we don't know the calling workflow's matrix
+      // Mark as incomplete to avoid false positive "Context access might be invalid" warnings
+      const matrixContext = new DescriptionDictionary();
+      matrixContext.complete = false;
+      return matrixContext;
+    }
   }
 
   return undefined;