Pull ref from Gitaly if not present in Redis

What does this MR do and why?

• Call Gitaly RefExists RPC when there is a branch ref not found in Redis cache. This will work as a fallback to validate the latest repository state.
• Track when we trigger the fallback to investigate why the cache is stale in some scenarios.

References

• Discussion 1: #348565 (comment 2399631267)
• Discussion 2: #348565 (comment 2391039320)

How to set up and validate locally

It's not possible (or very difficult) to replicate the stale cache locally. We should rely on the Gitaly fallback and their tests.

MR acceptance checklist

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Related to #348565 (closed)