* fix(core): always create system message even with _init_with_no_messages
When _init_with_no_messages=True (used by agent import flows), the agent
was created with message_ids=None. If subsequent message initialization
failed, this left orphaned agents that crash when context window is
calculated (TypeError on message_ids[1:]).
Now the system message is always generated and persisted, even when
skipping the rest of the initial message sequence. This ensures every
agent has at least message_ids=[system_message_id].
Fixes Datadog issue 773a24ea-eeb3-11f0-8f9f-da7ad0900000
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Co-Authored-By: Letta <noreply@letta.com>
* fix(core): clean up placeholder messages during import and add test
Delete placeholder system messages after imported messages are
successfully created (not before), so agents retain their safety-net
system message if import fails. Also adds a test verifying that
_init_with_no_messages=True still produces a valid context window.
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Co-Authored-By: Letta <noreply@letta.com>
* fix(core): add descriptive error for empty message_ids in get_system_message
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Co-Authored-By: Letta <noreply@letta.com>
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Co-authored-by: Letta <noreply@letta.com>
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