feat: make SPARQL endpoint path configurable

[stefanavesand]

Summary

Adds a configurable sparqlEndpointPath field to ConnectionConfig, allowing users to specify the SPARQL query endpoint path for their triple store. Defaults to "/sparql" for full backward compatibility.

Closes #23

Motivation

Graph Explorer currently hardcodes /sparql as the SPARQL endpoint path. This works for Amazon Neptune and Blazegraph, but many popular triple stores use different paths:

Triple Store	SPARQL Path
Amazon Neptune	/sparql
Blazegraph	/sparql
Apache Jena Fuseki	/query
Virtuoso	/sparql
GraphDB	/repositories/{repo}

Without this change, users of Fuseki, GraphDB, and other non-standard-path stores cannot use Graph Explorer at all.

Changes

• packages/shared/src/types/index.ts — Add optional sparqlEndpointPath to ConnectionConfig
• packages/graph-explorer/src/connector/sparql/sparqlExplorer.ts — Use connection.sparqlEndpointPath ?? "/sparql" instead of hardcoded "/sparql"
• packages/graph-explorer/src/connector/fetchDatabaseRequest.ts — Forward sparql-endpoint-path header to proxy when set
• packages/graph-explorer-proxy-server/src/node-server.ts — Read sparql-endpoint-path header, default to "/sparql"
• packages/graph-explorer/src/modules/CreateConnection/CreateConnection.tsx — Add "SPARQL Endpoint Path" input field (shown only when SPARQL is selected)
• packages/graph-explorer/src/core/defaultConnection.ts — Add GRAPH_EXP_SPARQL_ENDPOINT_PATH to default connection schema

Backward compatibility

• The field is optional everywhere — existing connections without it behave identically to before
• Default value is "/sparql" in both the frontend connector and the proxy server
• The UI field only appears when SPARQL query language is selected
• No changes to Gremlin or OpenCypher paths

Testing

Tested with Apache Jena Fuseki (which uses /query) against a knowledge graph with 4.7M triples — schema synchronization, node exploration, and graph visualization all work correctly with the custom path.

[kmcginnes]

@stefanavesand Thank you for the contribution. It might take some time before I can give this a proper review.

When I do get around to it, my team and I will need to discuss if this is the approach we want to take. In our past discussions we decided on a bigger change to the connection UI that would simplify the user experience and add this feature in a more streamlined way.

• Overhaul add/update connection UI #1326

So, my main concern would be to evaluate if any of the changes in this PR hinder our future plans. That would be in addition to the typical code review and manual testing for any PR.