Add SSE backend test fake and table-drive HeaderForward regression test

[lorr1]

Motivation

PR #5301 fixed the per-session HTTP HeaderForward gap reported in #5289. The regression test TestHTTPSession_AppliesHeaderForwardToPostInitializeRequests (in pkg/vmcp/session/internal/backend/) drives a real MCP client through the production transport chain, but exercises only the streamable-http transport because that is the only transport our test fakes can speak.

createMCPClient (pkg/vmcp/session/internal/backend/mcp_session.go) supports both streamable-http and sse. The BuildHeaderForwardTripper wrap is applied to the shared base before the per-transport switch, so SSE is wired correctly today — but a future refactor that moves the wrap into the streamable-http arm only would silently re-break SSE (the exact failure mode of the original #5289 bug, just on a different branch).

What is needed

1. Extend the canonical fakeBackend test helper (in pkg/vmcp/session/internal/backend/mcp_session_capabilities_test.go) to support the SSE transport, OR add a sibling SSE fake in the same file. The fake must:

   • Serve GET /sse with Content-Type: text/event-stream.
   • Push the initial endpoint event so the mark3labs mcp-go SSE client knows where to POST commands.
   • Accept POST /messages (or whatever endpoint the client requests) for outbound JSON-RPC requests, recording inbound headers per method (same recording shape fakeBackend already has for streamable-http after PR Wire HeaderForward into vMCP per-session HTTP client #5301's second commit).
   • Push responses back over the SSE stream in event: message, data: {...}\n\n format.
   • Close cleanly when the test ends.

2. Table-drive TestHTTPSession_AppliesHeaderForwardToPostInitializeRequests over both transport types so the regression is enforced for SSE as well as streamable-http. Each table row asserts the post-initialize request carries the configured X-MCP-Toolsets header.

3. Apply the same table-drive to the overlap-precedence and AddHeadersFromSecret subtests that PR Wire HeaderForward into vMCP per-session HTTP client #5301 added, so both subbranches get equivalent coverage.

Out of scope

• This is a test-only enhancement; no production behavior change.
• Not blocked by and does not block vmcp/session: expose a pluggable backendConnector on NewSessionFactory #4929 (pluggable backendConnector) or Add proper SSE support for vMCP (beyond streamable-http session affinity) #4974 (vmcp-as-server SSE scaling).

Acceptance criteria

• A test SSE fake exists in pkg/vmcp/session/internal/backend/ that can serve a full initialize → tools/list → tools/call handshake to an mcp-go SSE client.
• Header recording works identically for both transports (same headersFor(method) accessor).
• TestHTTPSession_AppliesHeaderForwardToPostInitializeRequests runs as a table over both transports and passes for each.
• No flakes under -race.

Related: #5289, #5301, #5302.