SSE response mode no longer forced; 503 overflow fixed

Patch Changes

• af7e4ad: Stop forcing responseMode: "sse" on the HTTP handler and use the SDK default "auto" instead. Forcing "sse" put every response on an SSE stream, and those streams were not released: concurrent upstream streams went from ~10 before v4.0.0 to over 5000, exhausting the gateway connection pool and returning 503 reset reason: overflow on mcp.context7.com. Traffic and latency were unchanged over that period, so the growth was not load.

  With "auto" a request is answered with a single JSON body unless a handler emits a related message before its result, which upgrades that one exchange to SSE. No tool emits progress today, so modern-protocol responses are now plain JSON. The 2025-era legacy fallback is constructed without a responseMode and still streams over SSE, so it is unaffected.