Consolidate shared package into graph-explorer client

The `packages/shared` package currently contains types (`ConnectionConfig`, `QueryEngine`) and test utilities that are only imported by the `graph-explorer` client. The proxy server lists `@graph-explorer/shared` as a dependency but has zero actual imports from it.

Consider moving the shared package contents into `packages/graph-explorer` to simplify the monorepo structure. If the proxy server needs shared types in the future, the package can be re-extracted at that point.

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