Fix tests for SDK 2.0

[jongalloway]

Fix tests for SDK 2.0

Part of #481.

The test project (DotNetMcp.Tests) references ModelContextProtocol 1.4.0 and includes contract/conformance tests that may assert on changed surface.

Tasks

• Bump the test project to SDK 2.0.x (tracked with #SUB2).
• Update/remove tests that reference removed experimental Tasks types (#SUB3).
• Ensure any Tool fixtures provide InputSchema (now required on deserialization — SEP-2106).
• Update error-code assertions affected by #SUB6.
• Re-run the full suite and restore green.

Acceptance

• Full test suite passes against SDK 2.0.