chore: Add a Release Please manifest.

Release please seems to be starting at version 0.1.0 while we've already manually released 0.3.0.

I **think** checking in the .release-please-manifest.json with the version set should fix that.

The `.json` files in this change were created by running:

```
release-please bootstrap --release-type=python --initial-version=0.3.0 --bump-minor-pre-major --repo-url=amirh/python-genai
```

PiperOrigin-RevId: 709117873

.github/release-please.yml

@@ -1,3 +1,4 @@
 releaseType: python
 handleGHRelease: true
 bumpMinorPreMajor: true
+manifest: true

.release-please-manifest.json

@@ -0,0 +1,3 @@
+{
+  ".": "0.3.0"
+}

release-please-config.json

@@ -0,0 +1,13 @@
+{
+  "packages": {
+    ".": {
+      "changelog-path": "CHANGELOG.md",
+      "release-type": "python",
+      "bump-minor-pre-major": true,
+      "bump-patch-for-minor-pre-major": false,
+      "draft": false,
+      "prerelease": false
+    }
+  },
+  "$schema": "https://raw.githubusercontent.com/googleapis/release-please/main/schemas/config.json"
+}