🔴 Required Information

Describe the Bug:
A ValueError is raised in google-adk version 2.2.0 when running an agent hosted on Agent Runtime -- formerly known as Agent Engine / Reasoning Engine (using Vertex AI managed sessions) and querying it from Gemini Enterprise App.

The fully qualified session resource path (which contains slashes /) was passed to the session service. This fails the strict session ID format check (_validate_session_id) added to vertex_ai_session_service.py in 2.2.0 (commit b2916c7).

Steps to Reproduce:

1. Deploy a Python ADK agent using google-adk version 2.2.0 on Google Cloud Agent Runtime.
2. Query the agent via the Gemini Enterprise app.
3. The platform sends a fully qualified session path.
4. The server throws a ValueError, seen in Google Cloud Logging.

Expected Behavior:
The SDK should either parse/sanitize the fully qualified resource name to extract the short session ID (last segment after /) before validation, or the validator should support full resource paths when invoked by the platform runner.

Observed Behavior:
The server crash logs show:

ValueError: Invalid session_id 'projects/my_project_number/locations/global/collections/default_collection/engines/my_app/sessions/my_session_id': must match ^[A-Za-z0-9_-]+$.

Environment Details:

• ADK Library Version (pip show google-adk): 2.2.0
• Desktop OS: macOS
• Python Version (python -V): Python 3.11

Model Information:

• Are you using LiteLLM: No
• Which model is being used: gemini-3.5-flash

🟡 Optional Information

Regression:
Yes. This worked in google-adk versions prior to 2.2.0 where the _validate_session_id check was not yet present.
The regression was introduced in the commit: b2916c7 (related to #5580).

Logs:

File "/home/myuser/.local/lib/python3.11/site-packages/google/adk/cli/fast_api.py", line 797, in json_generator
  async for chunk in output:
File "/home/myuser/.local/lib/python3.11/site-packages/vertexai/agent_engines/templates/adk.py", line 1342, in streaming_agent_run_with_events
  session = await session_service.get_session(
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/myuser/.local/lib/python3.11/site-packages/google/adk/sessions/vertex_ai_session_service.py", line 181, in get_session
  _validate_session_id(session_id)
File "/home/myuser/.local/lib/python3.11/site-packages/google/adk/sessions/vertex_ai_session_service.py", line 58, in _validate_session_id
  raise ValueError(
ValueError: Invalid session_id 'projects/my_project_number/locations/global/collections/default_collection/engines/my_app/sessions/my_session_id': must match ^[A-Za-z0-9_-]+$.

Additional Context:
The commit b2916c7 added _validate_session_id to verify session IDs against ^[A-Za-z0-9_-]+$. However, the ADK runner forwards the full platform path to Vertex AI Session Service, bypassing any extraction, which leads to this crash in hosted environments.

How often has this issue occurred?:

• Always (100%) when queried from platforms sending fully qualified paths (like Gemini Enterprise).