Fix protocol version tag for stateless server spans

[luisangelrod]

Summary

• Use the per-request protocol version when tagging server activities before session-level negotiation state is populated
• Add stateless HTTP coverage for the 2026-07-28 protocol version tag

Fixes #1800

Testing

• Focused stateless protocol-version activity test: 1 passed
• ModelContextProtocol.Tests DiagnosticTests on net10.0: 3 passed
• MapMcpStatelessTests on net10.0: 32 passed, 20 skipped as expected

[jeffhandley]

Thanks for submitting this, @luisangelrod! Tagging @tarekgh for review.