fix: add_episode creates the episode when a supplied uuid is not found

[brentkearney]

Summary

Fixes #1646: add_episode currently raises NodeNotFoundError when given a uuid that isn't in the graph yet. This change makes it create the episode under the supplied uuid instead, so callers can pre-assign episode ids.

Behavior after the change:

• uuid exists in the graph → reprocess that episode (unchanged)
• uuid supplied but not found → create the episode with the supplied uuid (was: crash)
• uuid omitted → generate one (unchanged)

The only code path that changes is the one that currently raises, so no existing caller is affected. Creating under a supplied uuid matches the MERGE-on-uuid semantics the save path already uses.

This also gives callers the primitive requested in #1283 (reference episodes in previous_episode_uuids before their processing completes — just pre-assign the ids) and covers the user-provided-id part of #961.

Changes

• graphiti_core/graphiti.py — add_episode catches NodeNotFoundError on the uuid lookup and falls through to creating the episode with the supplied uuid; docstring updated
• mcp_server/src/graphiti_mcp_server.py — add_memory docstring updated to document the new uuid semantics
• tests/test_add_episode_uuid.py — tests for all three uuid paths (existing uuid reprocesses, new uuid creates with that uuid, omitted uuid generates)

Closes #1646

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[brentkearney]

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