Fix HTTP fallback error handling - #1791

Structured HTTP 400 responses containing non-modern JSON-RPC errors now fall back from server/discover to initialize for both Streamable HTTP and AutoDetect.

AutoDetect now preserves HTTP semantics for structured non-modern JSON-RPC errors returned with non-400 statuses such as 401, 403, and 500. These responses remain HttpRequestExceptions with their status and body, without attempting initialize or deprecated SSE fallback. Streamable HTTP and AutoDetect share the same status/error-code classification so their behavior remains consistent.

The first fix is intentionally simple despite the architectural alternatives discussed in #1790. #1692 initially added a local transport validation exception when a modern response carried Mcp-Session-Id, together with a catch that prevented fallback for that exception. A later commit in the same PR updated the spec behavior to ignore unexpected session IDs and removed the local exception, but inadvertently left the catch behind. Removing that stale catch restores the intended fallback behavior; no provenance architecture or exception subclass is needed.

Validation:

• dotnet build --nologo passed.
• July2026ProtocolFallbackTests: 20/20 passed on net472, net8.0, net9.0, and net10.0.
• July2026ProtocolHttpFallbackTests: 8/8 passed on net8.0, net9.0, and net10.0.
• Broader managed test runs from the implementation sessions passed. The conformance CLI was blocked only by the local Node v21.6.2 environment lacking fs.globSync; this was not a code regression.

Fixes #1790