fix(mcp): honor custom endpoint in StreamableHttpServerParameters

[YuqiGuo105]

Summary

Fixes #1196

When a user passed a full URL with a custom sub-path to StreamableHttpServerParameters (e.g. http://localhost:8080/mcp/stream), the MCP library's Utils.resolveUri() would call Java's URI.resolve() with the default endpoint "/mcp". Per RFC 3986, when the second argument is an absolute path (starts with /), it replaces the entire path of the base URI. The result was always http://localhost:8080/mcp — the custom sub-path was silently dropped — causing a 404 Not Found on every request against servers not exposed on /mcp.

Root Cause

HttpClientStreamableHttpTransport internally stores a baseUri and an endpoint, and uses Utils.resolveUri(baseUri, endpoint) for every request. The default endpoint is "/mcp". When the full custom URL was passed as the baseUri, URI.resolve("/mcp") stripped the custom suffix:

URI.create("http://localhost:8080/mcp/stream").resolve("/mcp")
  → http://localhost:8080/mcp   ❌

Fix

• Added an optional endpoint field (and matching Builder.endpoint() setter) to StreamableHttpServerParameters, following the same pattern as SseServerParameters.sseEndpoint.
• In DefaultMcpTransportBuilder, the base URL (host only, e.g. http://localhost:8080) is passed to HttpClientStreamableHttpTransport.Builder, and when the user has provided a custom endpoint path, it is passed to the library's own .endpoint() method.
  This makes resolveUri work correctly:

  URI.create("http://localhost:8080").resolve("/mcp/stream")
    → http://localhost:8080/mcp/stream   ✅
  

• When no endpoint is set, the library's default "/mcp" is preserved — full backward compatibility.

Changes

File	Change
core/src/main/java/…/mcp/StreamableHttpServerParameters.java	Add endpoint field, constructor param, getter, and Builder.endpoint()
core/src/main/java/…/mcp/DefaultMcpTransportBuilder.java	Conditionally call .endpoint() on the transport builder
core/src/test/…/mcp/StreamableHttpServerParametersTest.java	New — 11 unit tests covering constructor and builder
core/src/test/…/mcp/DefaultMcpTransportBuilderTest.java	New — 8 unit tests verifying endpoint wiring and URI resolution

Testing

All 22 new tests pass. Existing core tests unaffected.