chore: add explicit permissions to release-please workflow (#513)

## Summary

Adds explicit `contents: write` and `pull-requests: write` permissions
to the `release-please` job. These are required for the release-please
action to create release PRs and GitHub releases. Without explicit
permissions, the job relies on the repository/org default `GITHUB_TOKEN`
permissions, which may be insufficient if defaults are tightened to
read-only.

Downstream jobs (release-client, release-server, provenance, etc.)
already have their own explicit permissions blocks and are unaffected by
this change.

## Review & Testing Checklist for Human

- [ ] **Verify no other implicit permissions are needed by the
`release-please` job.** Adding an explicit job-level `permissions` block
restricts the token to *only* the listed permissions, revoking any
previously inherited defaults. If the release-please action in this repo
needs anything beyond `contents` and `pull-requests` (e.g., `id-token:
write`), it will break.
- [ ] After merging, monitor the next `release-please` workflow run
(triggered by a push to `main`) to confirm it still creates/updates
release PRs successfully.

### Notes

This is part of a batch update across all `launchdarkly-sdk`-tagged
repositories whose release-please workflows were missing explicit
permissions on their default branch.

Link to Devin session:
https://app.devin.ai/sessions/a83b6e4f4fa14b96b859cfb50755a2c1
Requested by: @kinyoklion

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk configuration change, but it can break releases if
`release-please` requires additional permissions beyond `contents` and
`pull-requests` under restricted tokens.
> 
> **Overview**
> **Hardens the `release-please` GitHub Actions workflow** by adding an
explicit job-level `permissions` block for `release-please` (`contents:
write` and `pull-requests: write`).
> 
> This removes reliance on repository/org default `GITHUB_TOKEN`
permissions and ensures the job can create/update release PRs and GitHub
releases when defaults are tightened.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
16f70bb. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Author: kinyoklion

.github/workflows/release-please.yml

@@ -7,6 +7,9 @@ name: release-please
 jobs:
   release-please:
     runs-on: ubuntu-22.04
+    permissions:
+      contents: write
+      pull-requests: write
     outputs:
       package-client-released: ${{ steps.release.outputs['libs/client-sdk--release_created'] }}
       package-client-tag: ${{ steps.release.outputs['libs/client-sdk--tag_name'] }}