Add MustBePositiveOrInfinite assertion

[feO2x]

Context

Parent issue #162 (point 6) identifies two BrilliantMessaging guards that accept either a positive TimeSpan or the standard .NET infinite-timeout sentinel. Expressing this with a generic predicate obscures the platform convention.

Proposed API

timeout.MustBePositiveOrInfinite();

The assertion should return the original value when it is greater than TimeSpan.Zero or exactly equals Timeout.InfiniteTimeSpan. It should reject zero and every other negative value. Default and custom-exception-factory overloads should follow the existing TimeSpan sign-guard conventions.

Scope

• Add MustBePositiveOrInfinite for TimeSpan on all supported target frameworks.
• Cover success, failure, exception-contract, and custom-factory behavior with automated tests.
• Expose the assertion through the source-code-transformation whitelist and verify its exported form.
• Update assertion documentation and package release notes.