fix(ci): keep an optional branch input on shared-ci

ci.yml forwards `branch` to shared-ci.yml, but that input was just removed,
so the reusable-workflow call failed at startup on every pull_request run.

Re-add `branch` as optional (default empty = the triggering ref) and forward
it from ci.yml without the `|| 'master'` fallback. PR CI now checks out the
PR ref instead of master, release still uses the dispatched ref, and daily CI
can still target the v4.x maintenance branch.

Author: lucasmcdonald3

.github/workflows/ci.yml

@@ -9,16 +9,16 @@ on:
   workflow_call:
     inputs:
       branch:
-        description: "Branch to checkout"
+        description: "Ref to check out. Empty (the default) uses the ref that triggered the run."
         required: false
-        default: master
+        default: ''
         type: string
 
 jobs:
   shared-ci:
     uses: ./.github/workflows/shared-ci.yml
     with:
-      branch: ${{ inputs.branch || 'master' }}
+      branch: ${{ inputs.branch }}
   pr-ci-all-required:
     if: always()
     needs:

.github/workflows/shared-ci.yml

@@ -3,6 +3,11 @@ name: Shared CI Tests
 on:
   workflow_call:
     inputs:
+      branch:
+        description: 'Ref to check out. Empty (the default) uses the ref that triggered the run.'
+        required: false
+        default: ''
+        type: string
       test-published-packages:
         description: 'Test against published packages instead of checked out code'
         required: false
@@ -34,6 +39,7 @@ jobs:
         # Always need repo for test scripts and configuration, even when testing published packages
         uses: actions/checkout@v4
         with:
+          ref: ${{ inputs.branch }}
           fetch-depth: 0
           submodules: true