docs: update unit tests execute command (#2074)

## Description

update unit tests execute command in DEVELOPER.md to exclude integration
tests

## PR Checklist

- [x] Make sure you reviewed

[CONTRIBUTING.md](https://github.com/googleapis/genai-toolbox/blob/main/CONTRIBUTING.md)
- [ ] Make sure to open an issue as a

[bug/issue](https://github.com/googleapis/genai-toolbox/issues/new/choose)
  before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)
- [x] Make sure to add `!` if this involve a breaking change

Co-authored-by: Averi Kitsch <akitsch@google.com>

DEVELOPER.md

@@ -109,7 +109,7 @@ golangci-lint run --fix
 Execute unit tests locally:
 
 ```bash
-go test -race -v ./...
+go test -race -v ./cmd/... ./internal/...
 ```
 
 ### Integration Tests