[tonmoy-y]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: InMemoryMemoryService loses sessionless memories when a session ID equals its fallback bucket name #7711

Problem:
InMemoryMemoryService.add_events_to_memory() stores events added without a session ID under the string __unknown_session_id__. InMemorySessionService accepts that literal as an ordinary session ID, so add_session_to_memory() for such a session replaces the sessionless bucket and silently removes earlier sessionless memories.

Solution:
Key the private sessionless bucket by None instead of a string. Caller-provided session IDs are always str, so they can no longer alias it. Ordinary session IDs, replacement and deletion behave as before, and empty/omitted IDs still map to the same bucket. Only InMemoryMemoryService is changed; the SQLite and MongoDB services use the same string constant but are out of scope for this issue.

Testing Plan

Unit Tests:

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

Added test_add_session_to_memory_keeps_sessionless_memories (parametrized with ordinary-session and __unknown_session_id__). The __unknown_session_id__ case fails on main and passes with this change.

$ pytest tests/unittests/memory -q
178 passed, 1 warning

Manual End-to-End (E2E) Tests:

Ran the reproduction script from #7711 (Python 3.12):

before: ordinary-session: before=1, after=1
        __unknown_session_id__: before=1, after=0
after:  ordinary-session: before=1, after=1
        __unknown_session_id__: before=1, after=1

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.

Additional context

@widechaos commented on #7711 asking to take it on. I found no PR or branch for it and no maintainer reply, so I opened this; happy to close it in favour of theirs if they already have one in progress. Tested on Python 3.12 only; full supported-version tox run not done.

[widechaos]

Please keep this PR open. I have a local candidate for #7711, but I haven't submitted a PR. I'll leave the implementation with you and won't open a competing one. I haven't independently reviewed or tested your patch.