test(mcp): add regression test for nil ClientInfo panic

[manusa]

Adds a regression test for the nil ClientInfo panic fixed in #844 (originally reported in #842).

The existing TestFallsBackToServerPrefixWhenNoClientInfo test uses WithEmptyClientInfo() which sets ClientInfo to an empty (but non-nil) *Implementation{}. This doesn't cover the actual panic scenario where ClientInfo is nil — which happens when a client omits clientInfo from the initialize request entirely.

Since the go-sdk client always sets clientInfo and panics if the implementation is nil, we can't reproduce this through the standard test client. Instead, the new test sends raw HTTP JSON-RPC requests with an initialize payload that omits clientInfo, simulating a non-compliant client. This approach is inspired by the CallToolRaw pattern used in podman-mcp-server for testing raw JSON-RPC interactions.

The MCP spec mandates that clientInfo is sent during initialization, but some clients don't follow this, so we should handle it gracefully.

[manusa]

Note: overlap with #859

PR #859 introduces test.McpRawPost — a reusable helper for sending raw JSON-RPC requests to MCP HTTP endpoints, bypassing the go-sdk client's automatic normalization. This is the same pattern used inline in this PR.

If #859 merges first, this PR can be rebased and simplified by replacing the inline HTTP request construction with test.McpRawPost calls:

// Raw initialize (no clientInfo)
initResp := test.McpRawPost(s.T(), httpServer.URL+"/mcp", "",
    `{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"protocolVersion":"2025-03-26"}}`)
defer initResp.Body.Close()
_, _ = io.ReadAll(initResp.Body)
sessionID := initResp.Header.Get("Mcp-Session-Id")

// Raw tool call
toolResp := test.McpRawPost(s.T(), httpServer.URL+"/mcp", sessionID,
    `{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"pods_list","arguments":{}}}`)
defer toolResp.Body.Close()

This eliminates the duplicated HTTP boilerplate (Content-Type, Accept headers, error handling) while keeping the custom httptest.Server setup this test needs for User-Agent stripping.