[subhashpolisetti]

Fixes #1274

Motivation and Context

McpAsyncServerExchange#listRoots() follows nextCursor until it is null. When a client answers roots/list with "nextCursor": "", the server requests the next page with "cursor": "", and a client that keeps answering that way makes listRoots() never complete. McpSyncServerExchange#listRoots() blocks on the same Mono, so it hangs the calling thread.

#954 fixed this on the client for listTools, listResources, listResourceTemplates and listPrompts. This change applies the same guard, line for line, to listRoots(), the last aggregate list method in mcp-core without it:

.expand(result -> {
	String next = result.nextCursor();
	return (next != null && !next.isEmpty()) ? this.listRoots(next) : Mono.empty();
})

McpSyncServerExchange#listRoots() delegates to this method, so it needs no change.

How Has This Been Tested?

Added testListRootsStopsOnEmptyNextCursor to McpAsyncServerExchangeTests: the first page returns nextCursor: "", and the test checks that only that page is returned and that roots/list is sent once. It fails on main (a second request goes out with an empty cursor) and passes with this change.

./mvnw -pl mcp-core test -Dtest=McpAsyncServerExchangeTests,McpSyncServerExchangeTests
./mvnw clean verify
./mvnw -pl mcp-test -am -Pjackson2 test

• McpAsyncServerExchangeTests 38/38, McpSyncServerExchangeTests 27/27
• clean verify: BUILD SUCCESS (mcp-core 506, mcp-json-jackson3 45, mcp-json-jackson2 45, mcp-test 1357; 1 existing assumeTrue skip in StdioMcpAsyncClientTests)
• -Pjackson2: BUILD SUCCESS, same counts

Breaking Changes

None. A non-empty nextCursor is still followed; an empty one now ends pagination, as it does on the client.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

• The same code is on 2.1.x, 2.0.x and 1.1.x. fix: stop pagination on empty cursors #954 is already on 2.1.x, so this would sit next to it if you backport.
• This only changes listRoots(). Reject listRoots when client lacks roots capability #1070 and Reject listRoots when client lacks roots capability #1137 add capability checks to listRoots(String cursor) in the same class; both merge cleanly with this branch.