fix: add vars context to composite action schema

The action manifest schema (`action_yaml.json`) was missing `vars` from
all composite step context definitions. This caused the runner to reject
`${{ vars.* }}` expressions in composite actions at parse time with:

    Unrecognized named-value: 'vars'

The workflow schema (`workflow-v1.0.json`) already includes `vars` in all
equivalent step-level contexts. This was a schema-only gap — the runner
properly populates the `vars` context at runtime via `ExpressionValues`.

Added `vars` to:
- `step-env`
- `string-steps-context`
- `boolean-steps-context`
- `step-if`
- `step-with`
- `output-value`

Fixes #4311

src/Runner.Worker/action_yaml.json

@@ -1,4 +1,4 @@
-{
+{
     "definitions": {
         "action-root": {
             "description": "Action file",
@@ -163,6 +163,7 @@
         "output-value": {
             "context": [
                 "github",
+                "vars",
                 "strategy",
                 "matrix",
                 "steps",
@@ -193,6 +194,7 @@
             "context": [
                 "github",
                 "inputs",
+                "vars",
                 "strategy",
                 "matrix",
                 "steps",
@@ -207,6 +209,7 @@
             "context": [
                 "github",
                 "inputs",
+                "vars",
                 "strategy",
                 "matrix",
                 "steps",
@@ -221,6 +224,7 @@
             "context": [
                 "github",
                 "inputs",
+                "vars",
                 "strategy",
                 "matrix",
                 "steps",
@@ -238,6 +242,7 @@
             "context": [
               "github",
               "inputs",
+              "vars",
               "strategy",
               "matrix",
               "steps",
@@ -256,6 +261,7 @@
             "context": [
                 "github",
                 "inputs",
+                "vars",
                 "strategy",
                 "matrix",
                 "steps",