Fix nested path URI resolution

[Qshuai0213]

Summary

Fixes #257

Utils.resolveUri incorrectly handled nested base paths — relative endpoints with a leading slash (e.g. /api/v1) would replace the base path entirely instead of being appended to it. This affected HttpClientSseClientTransport and HttpClientStreamableHttpTransport when the base URL had a non-root path (e.g. https://example.com/some).

Root cause

URI.resolve() follows RFC 3986 §5.2.3: when the relative reference starts with /, it replaces the entire path of the base URI. So URI.create("https://example.com/some").resolve("/path") yields https://example.com/path, dropping /some.

Fix approach

• Ensure the base URL always has a trailing slash (e.g. /root → /root/)
• Strip leading slashes from the endpoint (e.g. /api/v1 → api/v1)
• URI.resolve() then performs path concatenation (/root/ + api/v1 = /root/api/v1), consistent with RFC 3986
• Absolute endpoint validation behavior is preserved unchanged

Testing

• ./mvnw.cmd -pl mcp-core -Dtest=UtilsTests test
• ./mvnw.cmd -pl mcp-core test
• Added parameterized test cases covering: nested base paths, leading/trailing slashes, and query strings

[Qshuai0213]

@chemicL Hi, since you've looked into this issue in both #257 and #701, could you take a look at this PR when you have time? Happy to adjust the approach if needed. Thanks!