fix: dedupe identical session snapshots

[vicary]

Summary

• Adds a per-session in-memory cache that stores the last-seen snapshot content for each session ID.
• Before ingesting a snapshot event, the handler compares the incoming content against the cached value; identical snapshots are silently skipped, eliminating redundant Graphiti ingestion calls.
• Adds unit tests covering: first snapshot always ingested, duplicate snapshot skipped, distinct sessions tracked independently, and changed snapshot after duplicate is re-ingested.

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Note

Low Risk
Low risk behavior change limited to session.idle snapshot ingestion, reducing redundant addEpisode calls. Main risk is unintentionally skipping snapshots if downstream relied on duplicates, but dedupe only applies to exact content matches and only after a successful save.

Overview
Prevents redundant Graphiti ingestion on session.idle by caching the last successfully saved snapshot body per session and skipping client.addEpisode when the newly generated snapshot is identical.

Adds unit tests covering first-snapshot behavior, skipping identical subsequent snapshots, re-saving when content changes, and ensuring failed addEpisode attempts don’t update the dedupe cache (so retries still occur).

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