feat: do not fail on non-existent namespace in Rust core

[yquansah]

Makes a variety of changes.

1. Do not return errors from parsing the HTTP response which has a trickle effect to the engine
2. Modify tests to account for error case when passing in a 'nonexistent' flag

Completes FLI-730

[linear]

FLI-730 Bug: panic on notfound namespace

https://github.com/flipt-io/client-sdks/actions/runs/7120260245/job/19387131032

"grpc.service": "flipt.evaluation.DataService", "grpc.method": "EvaluationSnapshotNamespace", "peer.address": "127.0.0.1:52166", "error": "rpc error: code = NotFound desc = getting flags: namespace \"`CXN3V\" not found", "grpc.code": "NotFound", "grpc.time_ms": 0.426}
76: ...

127: exec bundle exec rspec ERROR: process "bundle exec rspec" did not complete successfully: exit code: 255
127: [0.58s] Calling `DidYouMean::SPELL_CHECKERS.merge!(error_name => spell_checker)' has been deprecated. Please call `DidYouMean.correct_error(error_name, spell_checker)' instead.
127: [1.27s] thread '<unnamed>' panicked at flipt-engine/src/lib.rs:189:50:
127: [1.27s] called `Result::unwrap()` on an `Err` value: Whatever { source: None, message: "failed to deserialize response into document: err missing field `namespace` at line 1 column 81", backtrace: Backtrace(()) }

[markphelps]

looks great! ty

[yquansah]

@markphelps ruby.... 😞