Preserve URL path when proxy constructs database endpoint URLs

[kmcginnes]

Description

The proxy server used absolute paths in new URL() calls (e.g., new URL("/sparql", baseUrl)) which replaced the entire base URL path. This broke databases with non-root endpoints like BlazeGraph (/blazegraph/namespace/kb/sparql).

• Extract resolveEndpointUrl helper that ensures a trailing slash on the base URL before resolving relative endpoint paths
• Use a TypeScript template literal type constraint to prevent passing leading-slash paths at compile time
• Add tests for path preservation across proxy endpoints, connection normalization, environment variable parsing, and IAM header preservation

Validation

• All 1695 tests pass, all checks (lint, format, types) pass
• End-to-end verified with finch compose: Graph Explorer proxy → BlazeGraph at /blazegraph/namespace/kb/sparql returns real SPARQL results
• TypeScript enforces that resolveEndpointUrl rejects absolute paths ("/sparql" → compile error)

Related Issues

• Fixes Proxy server strips URL path from Graph Connection URL, breaking databases with non-root SPARQL endpoints #1752

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have verified pnpm checks passes with no errors.
• I have verified pnpm test passes with no failures.
• I have covered new added functionality with unit tests if necessary.
• I have updated documentation if necessary.