fix(bot-mode): canonical bot DMs always follow the profile's current config

[curator8888]

Fixes the mirror image of #89497: bot DMs stuck on a stale provider pin.

Summary

Bot-Mode canonical chats (the ONE forever DM per bot) and room plumbing sessions are plugin-owned scratch conversations. They are now created with an explicit follow_profile_config contract, persisted in the session row's model_config, so session.resume rebuilds from the member profile's CURRENT config instead of restoring the stored model/provider pin from an old row.

That stale pin is what left bot DMs stuck on a dead provider (e.g. "out of Nous credits" after the profile was switched to ollama-cloud) while the same bot worked fine in rooms — the mirror image of the room-plumbing bug (#89497 class). Normal 1:1 user chats keep the stored-runtime restore: opening an older chat must show the model it actually used.

How to Test

1. Configure a bot profile with model A (e.g. a paid Nous model), open its Bot Chat DM, send a message.
2. Switch the profile to model B (e.g. ollama-cloud/glm-5.1).
3. Reopen the DM. Before this fix: resume restores the stored model A pin → "out of Nous credits" / HTTP 404. After: resume uses the profile's current model B.

Verification

• Backend: _stored_session_runtime_overrides returns {} for rows carrying the marker (dict and JSON model_config shapes); unmarked rows still restore stored runtime; _ensure_session_db_row persists the marker only when the contract is set.
• Desktop: createCanonicalChat and ensureGroupChatSession both send follow_profile_config: true on session.create.
• Tests: 16/16 test_custom_provider_session_persistence.py, 24/24 with test_session_resume_db_ownership.py, 57/57 desktop group-chat tests.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective
• New and existing unit tests pass locally

[curator8888]

Superseded by #90343 — the DM fix was rebased into the room-plumbing PR so both bot-session contracts (room_plumbing + follow_profile_config) land as one coherent change.