Dispose the POST response in SseClientSessionTransport to stop leaking a connection per message

[yalcinfu22]

Fixes #1840

What

One-word change in SseClientSessionTransport.SendMessageAsync: the HttpResponseMessage
returned for each POSTed JSON-RPC message is now wrapped in using. A deterministic
regression test is included.

Why

McpHttpClient.SendAsync sends every request with HttpCompletionOption.ResponseHeadersRead,
so the underlying connection stays checked out until the response is consumed or disposed.
The POST path never did either on the success path (a 202 Accepted whose body is unused),
so every sent message stranded one ESTABLISHED connection — never returned to the pool,
never reused, reclaimed only by GC or an idle timeout. Measurements and the full analysis
are in #1840 (per client: 1 live SSE connection + one stranded connection per POST —
initialize, notifications/initialized, tools/list).

The surrounding code already does this correctly: the SSE GET response is wrapped in
using var response in the same file, and the failure path reads the body before throwing.
Only the success path was missing the dispose.

Regression test

HttpClientTransportTests.SendMessageAsync_Disposes_Response_On_Success drives the SSE
transport through the public HttpClientTransport.ConnectAsync + SendMessageAsync path
against the existing MockHttpHandler. The mocked POST response carries content that
records its own disposal, and the test asserts the response has been disposed by the time
SendMessageAsync returns. No sockets, no GC, no timing dependence: neither
HttpResponseMessage nor HttpContent has a finalizer, so nothing but the transport's
explicit dispose can set the flag.

dotnet test tests/ModelContextProtocol.Tests/ --filter "FullyQualifiedName~SendMessageAsync_Disposes_Response_On_Success"

• Without the fix (parent commit's SseClientSessionTransport.cs): fails on every
  target framework — net10.0, net9.0, net8.0, and net472.
• With the fix: passes on all four TFMs, in both Debug and Release. The full
  HttpClientTransportTests class is 17/17 on each TFM.
• Full ModelContextProtocol.Tests suite: green locally on net10.0 (2357 passed,
  0 failed, 6 skipped — the tests requiring external credentials or Docker).
  ModelContextProtocol.AspNetCore.Tests (which exercises the SSE client transport
  against a real in-memory Kestrel server) is also green locally on net10.0, net9.0,
  and net8.0 (615 passed, 0 failed, 30 skipped each).

Notes

• The failure path is unaffected: the body is read and the HttpRequestException is fully
  materialized from StatusCode/ReasonPhrase/body before the using scope unwinds; the
  exception holds no reference to the response.
• The full solution builds cleanly with TreatWarningsAsErrors enabled.

🤖 Generated with Claude Code

[yalcinfu22]

Per CONTRIBUTING.md's "tests are included for new features or bug fixes": added a deterministic regression test, HttpClientTransportTests.SendMessageAsync_Disposes_Response_On_Success. It fails on the parent commit on all four target frameworks (net10.0, net9.0, net8.0, net472 — the POST response is never disposed on the success path) and passes with the fix on all four. Description updated with the command and results.