ci: declare workflow-level contents: read on 6 workflows

Pins the default GITHUB_TOKEN to contents: read on the workflows in
.github/workflows/ that don't call a GitHub API beyond the initial
checkout. The other workflows in this directory are left implicit
because they need write scopes that a maintainer is better placed
to declare.

Motivation: CVE-2025-30066 (March 2025 tj-actions/changed-files
compromise) exfiltrated GITHUB_TOKEN from workflow logs. Per-workflow
caps bound runtime authority irrespective of repo or org default,
give drift protection if the default ever widens, and are credited
per-file by the OpenSSF Scorecard Token-Permissions check.

YAML validated locally with yaml.safe_load.

Signed-off-by: Arpit Jain <arpitjain099@gmail.com>

Author: arpitjain099

.github/workflows/check_diff.yml

@@ -33,6 +33,9 @@ on:
         description: 'Optional comma separated list of rustfmt config options to pass when running the feature branch'
         required: false
 
+permissions:
+  contents: read
+
 jobs:
   diff_check:
     runs-on: ubuntu-latest

.github/workflows/integration.yml

@@ -5,6 +5,9 @@ on:
       - main
   pull_request:
 
+permissions:
+  contents: read
+
 jobs:
   integration-tests:
     runs-on: ubuntu-latest

.github/workflows/linux.yml

@@ -5,6 +5,9 @@ on:
       - main
   pull_request:
 
+permissions:
+  contents: read
+
 jobs:
   test:
     runs-on: ubuntu-latest

.github/workflows/mac.yml

@@ -5,6 +5,9 @@ on:
       - main
   pull_request:
 
+permissions:
+  contents: read
+
 jobs:
   test:
     # https://help.github.com/en/actions/automating-your-workflow-with-github-actions/virtual-environments-for-github-hosted-runners#supported-runners-and-hardware-resources

.github/workflows/rustdoc_check.yml

@@ -5,6 +5,9 @@ on:
       - main
   pull_request:
 
+permissions:
+  contents: read
+
 jobs:
   rustdoc_check:
     runs-on: ubuntu-latest

.github/workflows/windows.yml

@@ -5,6 +5,9 @@ on:
       - main
   pull_request:
 
+permissions:
+  contents: read
+
 jobs:
   test:
     runs-on: windows-latest