fix(memory): atomic graph persistence and validate entity existence in create_relations

[AbhiPra24]

Summary

Fixes #4614 and #4457.

1. Atomic Persistence in saveGraph() (Fixes #4614)

• Problem: saveGraph() directly called fs.writeFile(this.memoryFilePath, ...), which truncates the file immediately upon opening. If the process is terminated mid-write, the memory file becomes corrupted or empty.
• Fix: Write serialized lines to a temporary file in the same directory (${this.memoryFilePath}.${randomBytes(16).toString('hex')}.tmp), followed by an atomic fs.rename over this.memoryFilePath. The temporary file is cleaned up if an error occurs.

2. Referential Integrity in createRelations() (Fixes #4457)

• Problem: createRelations() only checked for duplicate relations, accepting dangling relations between non-existent entities without validation.
• Fix: Validates that all referenced from and to entities exist in graph.entities prior to mutating the graph, throwing Entity with name <name> not found to match addObservations().

Verification

• Added regression tests in src/memory/__tests__/knowledge-graph.test.ts.
• npm run build and npm test passed (56/56 tests green).

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cc @olaservo for review when available. Thank you!