feat: Add flag overrides to the contract test services - #530

…thout the init key (#539)

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## Summary

Overrides only add to what the SDK reports: a flag that the override
layer does not hold evaluates as it does without an override source, and
the all-flags state holds every flag it holds without one, plus the
overridden flags. A client that has not initialized is where SDKs broke
this, and only when the store holds flags without being initialized, for
example a persistent store that holds data but no init key. The flag
overrides suite uses the SDK's in-memory store, which is empty until a
data source provides data, so it could not tell a read of the override
layer apart from a read of everything the store returns.

A new `flag overrides` group in the persistent data store suite
(`server_side_persistence_overrides.go`) runs when the test service has
the `flag-overrides` capability. It writes flags to the store without
the init key and gives the client a data source that never provides
data. Each test compares a client with an override source to a client
without one against the same store, so it holds whatever an SDK reports
for an uninitialized store:

- **a flag without an override evaluates as it does without an override
source**: an override file that holds a different flag leaves the
evaluation of a store flag unchanged (OVERRIDE Requirements 1.5.1 and
1.5.3).
- **an empty override source leaves the all-flags state unchanged**
(Requirement 1.2.2).
- **overrides add to the all-flags state and make it valid**: the state
holds exactly the flags of the client without a source, with override
values for the overridden keys, plus the flags that exist only in the
override layer, and it is valid (launchdarkly/sdk-specs#280,
Requirements 1.4.1.2 and 1.4.1.3).

The flag overrides suite gains **evaluate all flags returns only the
override flags as a valid state** in its uninitialized client group,
where the store is empty and every SDK reports only the override flags.

## Results

Run with `-enable-persistence-tests` against Redis.

| Test service | flag without an override | empty override source |
overrides add |
|---|---|---|---|
| Go `v7` | fails: served from the store | fails: valid state of the
store's flags | fails: the store's flags join the overrides |
| Go launchdarkly/go-server-sdk#491 | passes | passes | passes |
| Node `main` | passes | fails: valid state of the store's flags |
passes |
| Node launchdarkly/js-core#2132 first commit (`57498a9e`) | passes |
passes | fails: drops a store flag the state holds without overrides |
| Node launchdarkly/js-core#2132 | passes | passes | passes |
| Python (top of the launchdarkly/python-server-sdk#530 stack, sync and
async) | passes | passes | passes |
| Ruby (top of the launchdarkly/ruby-server-sdk#454 stack) | passes |
passes | passes |

The Java (launchdarkly/java-core#225) and .NET (`dotnet-server-sdk-v9`)
test services do not support persistent stores, so the group skips
there; the new uninitialized client test passes on both. The full Redis
persistent data store suite and the flag overrides suite pass against Go
#491 at `30d4f216` (131 tests run). `golangci-lint` reports no issues.

Go and Node fail these tests until launchdarkly/go-server-sdk#491 and
launchdarkly/js-core#2132 are merged, for test services whose CI runs
the persistent data store suite with this harness version.

[SDK-3419](https://launchdarkly.atlassian.net/browse/SDK-3419)
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<sub>Opened from termag</sub>


[SDK-3419]:
https://launchdarkly.atlassian.net/browse/SDK-3419?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

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---

> [!NOTE]
> **Overview**
> Adds harness coverage for **file flag overrides** when the SDK never
initializes but a **persistent store already holds feature data without
the `$inited` key**—a case the existing in-memory override suite could
not distinguish from “read everything from the store.”
> 
> A new **`flag overrides`** group under the server-side persistent data
store suite (`server_side_persistence_overrides.go`, gated on
`flag-overrides`) seeds Redis/Consul/Dynamo with flags, uses a blocking
unavailable data source, and compares clients with and without override
files via `uninitializedClient`. Scenarios assert: store flags not in
the override file behave like no override source; an empty override file
leaves **evaluate all flags** unchanged; overrides merge into all-flags
state, mark it **valid**, and apply override values while retaining
other store flags.
> 
> The standalone flag-overrides **`uninitialized client`** group gains
**evaluate all flags returns only the override flags as a valid state**
(empty store, overrides-only valid state). The persistent suite
registers the new group from `server_side_persistence_base.go`.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
09a8e02. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->