chore: merge release v1.28.0 to main

Merge https://github.com/google/adk-python/pull/5016

Syncs version bump and CHANGELOG from release v1.28.0 to main.

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/5016 from google:release/v1.28.0 4e05efb76c
PiperOrigin-RevId: 890122693

.github/.release-please-manifest.json

@@ -1,3 +1,3 @@
 {
-  ".": "1.27.4"
+  ".": "1.28.0"
 }

.github/release-please-config.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://raw.githubusercontent.com/googleapis/release-please/main/schemas/config.json",
-  "last-release-sha": "066fcec3e8e669d1c5360e1556afce3f7e068072",
+  "last-release-sha": "50209549206256abe5d1c5d84ab2b14dfdf80d66",
   "packages": {
     ".": {
       "release-type": "python",