Fix source generated proxy suitability testing

[Andrew Arnott (AArnott)]

Fix issues that only show up when the interceptor is not used

This enables a bunch of tests that were previously not running when the interceptor is suppressed. Once we run them in that context, many of them were failing. I fix those bugs here.

[Copilot]

Pull Request Overview

This PR fixes the testing of source-generated proxy suitability by correcting preprocessor directive placement and improving the logic for validating that proxies implement the correct set of interfaces.

• Reorganized preprocessor directives in test file to properly exclude source-generated tests when interceptors are not available
• Fixed interface validation logic to deduplicate required interfaces and properly check for assignability
• Removed trailing period from error message for consistency

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
test/StreamJsonRpc.Tests/JsonRpcProxyGenerationTests.cs	Corrected #if directive placement to properly scope the SourceGenerated test class when interceptors are unavailable
src/StreamJsonRpc/Reflection/ProxyInputs.cs	Removed trailing period from Requirements property string
src/StreamJsonRpc/Reflection/ProxyBase.cs	Enhanced proxy interface validation by deduplicating required interfaces and fixing the assignability check logic