Change ResourceCache to use OrderedDict for performance, update tests.

[brian-hussey]

… to be more rigourous.

🐛 Bug-fix PR

Before opening this PR please:

1. make lint - passes ruff, mypy, pylint
2. make test - all unit + integration tests green
3. make coverage - ≥ 90 %
4. make docker docker-run-ssl or make podman podman-run-ssl
5. Update relevant documentation.
6. Tested with sqlite and postgres + redis.
7. Manual regression no longer fails. Ensure the UI and /version work correctly.

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📌 Summary

This change addresses #1614. It works in the suggested change, ensures tests work as expected and make the tests more robust for the new model.
It removes the need for the "last_accessed" property in the CacheEntry class/objects.

🔁 Reproduction Steps

Link the issue and minimal steps to reproduce the bug.

🐞 Root Cause

Implementation of the cache could be improved.

💡 Fix Description

Using the suggested OrderedDict approach and testing around that.

🧪 Verification

Check	Command	Status
Lint suite	make lint
Unit tests	make test
Coverage ≥ 90 %	make coverage
Manual regression no longer fails	steps / screenshots

📐 MCP Compliance (if relevant)

• Matches current MCP spec
• No breaking change to MCP clients

✅ Checklist

• Code formatted (make black isort pre-commit)
• No secrets/credentials committed

[brian-hussey]

One question on this, should the expires_at be updated now that we're not using last_accessed for this or should the expiry time still be based on when it was first cached?

[kevalmahajan]

Hi @brian-hussey,

I think the expires_at value should remain fixed from the point of insertion, rather than being refreshed upon each access. Refreshing the TTL on every get() could result in frequently accessed keys remaining in cache indefinitely, which would most probably undermine the intended purpose of the TTL.

For example, if the TTL is set to 3600 seconds and an item is accessed every 30 minutes, it could effectively never expire, even if the data becomes stale.

So, if we only update LRU position and keep TTL fixed, it will ensure hot items stay in cache longer than cold items (LRU working correctly) and all items still expire after their TTL (data freshness guaranteed)