fix: make prompt-template loading atomic and reload-safe

[Bashman10]

Summary

• Make prompt-template reloads atomic so failed loads preserve the active valid configuration.
• Reject prompt-template paths that escape the application directory.
• Add regression coverage for malformed reloads, invalid template structures, and path traversal.

Verification

• npx jest --config jest.config.ts src/ai/prompt-template.service.spec.ts --runInBand
• git diff --check

Notes

• The repository-wide lint and build checks currently fail on upstream main because of pre-existing, unrelated configuration and TypeScript errors.

Closes #652

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[MaryammAli]

LGTM

[MaryammAli]

LGTM