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!

[Selven81]

Thanks for this. I took the explicit "utf-8" on the write — it doesn't change
behaviour, but it makes the assumption visible, and none of the other PRs spell
it out.

On the relation validation: it's good work, but it addresses #4457, which is a
separate concern from persistence. Bundling it here makes both harder to review
in isolation — which I think is exactly why you opened #4696. I said as much in
the summary.

#4614 (comment)

[olaservo]

The atomic-write half of this landed via #4642. The createRelations entity validation is a separate behaviour change (#4457); if you rebase that part alone onto main as its own PR it can be reviewed on its own.