[fallintoplace]

Why are these changes needed?

What: Keep an explicitly supplied storage backend, even when it is empty.

Why: A length-aware storage can be falsy before its first event. MemoryStream would silently replace it with a new MemoryStorage, so events went to the wrong backend.

Implementation: Check storage is not None rather than truthiness. Add a regression test with an initially empty storage that implements __len__.

Related issue number

None.

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. (None needed.)
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

Local validation: reproduced the fallback behavior in isolation and syntax-checked the regression test. Full AG2 tests were not run because project dependencies are unavailable locally.

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[BichengWang]

The new regression test fails at this head: python -m pytest -q test/stream --ignore=test/stream/redis gives 1 failed, 48 passed. At test/stream/test_custom_storage.py:31, ModelMessage is transient, so the default stream intentionally filters it out before saving. The history assertion therefore gets [] even with the production fix.

Please use a persisted event such as ModelRequest([TextInput("hello")]), or explicitly set persist_all=True. I separately checked a persisted request with both persistence modes and history reuse: those checks fail on main and pass here, so the storage selection change itself addresses the reported bug.

[vvlrff]

@fallintoplace Thanks!