fix: update agent-server image tag to existing build

The previous image tag 2c1e72a-python (from PR #13120) did not exist in the
container registry because:

1. Commit 2c1e72a was a merge commit
2. The Agent Server workflow only built images for the pull_request event
   (targeting commit 3e81846), not for the push event on the merge commit

This caused the Changes tab in the local GUI to fail, as the sandbox couldn't
start with a non-existent image.

Updated to 50d8f1b-python which is a recent successful push build from
2026-03-04T13:58:07Z that includes the query parameter support from SDK PR #2249.

Co-authored-by: openhands <openhands@all-hands.dev>

openhands/app_server/sandbox/sandbox_spec_service.py

@@ -13,7 +13,7 @@ from openhands.sdk.utils.models import DiscriminatedUnionMixin
 
 # The version of the agent server to use for deployments.
 # Typically this will be the same as the values from the pyproject.toml
-AGENT_SERVER_IMAGE = 'ghcr.io/openhands/agent-server:2c1e72a-python'
+AGENT_SERVER_IMAGE = 'ghcr.io/openhands/agent-server:50d8f1b-python'
 
 
 class SandboxSpecService(ABC):