fix(storage/fs): various fixes and base test cases

[GeorgeMac]

This fixes are few things which did not work after the first pass moving eval snapshots into the existing fs/snapshot code.

There were a few missing fields on various types:

• Description on Rollout
• Value on Segment type Rollout
• The eval flag was not being appended from the flag set
• The eval snap rollout was not being append
• The eval snapshot namespaces map was not being initialized

I added a bunch of simply baseline unit tests which surfaced all of these things and then I fixed them in this PR.

Here is what the robots said:

• internal/storage/fs/snapshot.go: Initialized the Namespaces field in evaluation.EvaluationSnapshot within the EmptySnapshot function.
• internal/storage/fs/snapshot.go: Added evalSnapFlag to the Flags slice in the addDoc function.
• internal/storage/fs/snapshot.go: Included the Description field in core.Rollout within the addDoc function.
• internal/storage/fs/snapshot.go: Set the Value field in evaluation.EvaluationRolloutSegment within the addDoc function.
• internal/storage/fs/snapshot.go: Added evalSnapRollout to the Rollouts slice in evalSnapFlag within the addDoc function.

Improved error handling:

• internal/storage/fs/snapshot.go: Added a check for namespace existence in the GetEvaluationDistributions function, returning an error if the namespace is not found.

Updates to test data:

• internal/storage/fs/testdata/empty/features.yml: Added a namespace key to the empty features test data.
• internal/storage/fs/testdata/production/features.yaml: Added detailed production features including flags, variants, rules, and segments.