Reject roots listing before sending unsupported client requests

[karthiksenv]

A server calling listRoots against an uninitialized client or one without the roots capability currently sends an unsupported roots/list request. Add the same fail-fast checks used by sampling and elicitation, returning an IllegalStateException before contacting the session.

Fixes #1067.

Both listRoots overloads are covered by regression tests that assert no session interaction before or after subscription. Roots support with listChanged=false remains valid. The shared client/server integration test now asserts the local exception and a successful tool response instead of allowing an unasserted call to pass.

Validation on Windows with Temurin 17.0.19 and embedded Tomcat 11.0.2:

• Before the production fix: 41 exchange tests ran; the four new rejection cases failed because sendRequest was called.
• After the fix: mvnw.cmd -pl mcp-core -am clean test passed all 457 core tests.
• Streamable HTTP and SSE testRootsWithoutCapability passed with both Jackson 3 and Jackson 2 (two tests per backend).
• Formatting validation and git diff --check passed.
• Full mvnw.cmd verify: core and both Jackson modules passed; mcp-test ran 822 tests with 0 assertion failures and 12 errors caused by unavailable Docker / dependent class initialization. Conformance modules were skipped after that failure. This is not a fully green local CI run.
• A fork CI dispatch was attempted but GitHub returned workflow ci.yml not found on the default branch; the Ubuntu/Docker CI environment remains unverified.

This is an AI-assisted contribution requiring human review. The repository's AI submission threshold is not met; the exact required disclosure is included in disclosure.txt.

[soyeladice-svg]

AI-assisted review: the new integration assertion appears to check the wrong failure mode. listRoots() returns a Mono<ListRootsResult>, and the new guards return Mono.error(...); they do not throw synchronously when exchange.listRoots() is called. assertThatThrownBy(exchange::listRoots) only invokes the method reference and receives the Mono, so it should not observe the IllegalStateException unless the publisher is subscribed/blocked. Could this assertion consume the publisher (for example assertThatThrownBy(() -> exchange.listRoots().block())...) or use StepVerifier as the unit tests do? Otherwise the integration test does not actually pin the new fail-fast behavior.