Validate snapshot storage IDs

[GreyforgeLabs]

Summary

• Reject snapshot IDs and session IDs that are not single filename segments before building snapshot metadata paths.
• Propagate validation errors through snapshot and revert-history save/load/delete paths.
• Add coverage for snapshot ID traversal, session-history traversal, accepted UUID-like IDs, and rejected path segments.

Fixes PlatformNetwork/bounty-challenge#53119.

Validation

• cargo test -p cortex-snapshot
• cargo check -p cortex-snapshot
• cargo fmt --package cortex-snapshot --check
• git diff --check

[greptile-apps]

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