Remove legacy `tests/` vitest projects once migrated to `src/__tests__`

Several packages still configure `vitest.config.ts` to include a legacy `tests/**` directory instead of the standard `src/__tests__/**` convention, with a `TODO remove after __tests__ are deleted!` marker:

- `packages/modules/http/vitest.config.ts`
- `packages/utils/observable/vitest.config.ts`

Once tests in these packages are migrated to `src/__tests__`, remove the legacy `tests/**` include and this tracking issue can be closed.

Start with packages/modules/http/vitest.config.ts and packages/utils/observable/vitest.config.ts, then inspect each package's legacy tests/** directory and its TODO marker. Migrate those tests to src/__tests__, remove the legacy tests/** include from both Vitest configurations, and verify that the packages' test suites still run successfully.

Written by the indexing model from the issue text.