Allow Streamable HTTP GET reconnection

[lntutor]

Fixes #1239.

Summary

• release the stateful session GET guard when an unsolicited SSE request ends
• release the underlying transport GET/TCS guard for non-resumable sessions so the client can open a replacement GET stream
• preserve event-store-backed state so notifications continue to be persisted for Last-Event-ID replay while disconnected
• add transport-level and end-to-end HTTP regression coverage for sequential GET streams on the same session

This fixes reconnection after proxies/load balancers close an idle SSE connection. Server keep-alive comments can remain a separate hardening improvement; reconnecting no longer receives the permanent HTTP 400 described in the issue.

Tests

• StreamableHttpServerTransportTests (6 passed across target frameworks)
• GetStream_CanReconnectAfterPreviousRequestEnds (3 passed)
• existing resumability reconnect tests (6 passed)
• ASP.NET Core tests excluding the unavailable Node conformance executable (1,389 passed)
• dotnet build ModelContextProtocol.slnx (36 projects, 0 warnings)
• targeted dotnet format --verify-no-changes

The unfiltered ASP.NET Core run cannot execute conformance cases locally because node_modules/.bin/conformance is absent.