test(sessions): add unit tests for _session_util

[Koushik-Salammagari]

Link to Issue or Description of Change

Description of the change (no existing issue):

Problem:
The google.adk.sessions._session_util module had no dedicated unit-test
coverage. Its extract_state_delta helper contains prefix-routing logic
(app:, user:, temp:) that is used by session services to split a flat
state dict into app/user/session deltas, and a regression here would silently
corrupt persisted session state.

Solution:
Add a focused, test-only module covering both functions and their edge cases.
No production code is changed.

Coverage added:

• decode_model — None passthrough, decoding a dict into a model instance,
  and raising on invalid data.
• extract_state_delta — app:/user: prefixed keys routed to their buckets
  with the prefix stripped, temp: keys skipped entirely, unprefixed keys
  routed to the session bucket, empty and None state, and a mixed-key case.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

$ pytest tests/unittests/sessions/test__session_util.py -q
...
10 passed in 1.03s

Manual End-to-End (E2E) Tests:

Not applicable — this is a test-only change with no runtime/user-facing impact.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.