test: MCP tests fail when the developer's ~/.claude.json has MCP servers (HOME is not sandboxed) #1386

Found while running the full suite for the v0.12.4 release.

test/mcp/headers.test.ts (5 tests) and test/server/httpapi-mcp.test.ts ("serves status endpoint") fail on any machine whose real ~/.claude.json has an entry under mcpServers. MCP discovery reads that file from the real $HOME, so the discovered server is created alongside the test's own servers: it has no requestInit, which breaks the headers assertions, and it changes the status endpoint's result.

test/preload.ts sandboxes the XDG directories but not HOME, so other home-directory config sources may leak the same way.

Reproduced on main (93fe816). With HOME pointed at an empty temporary directory, the same three files pass (15/15).

Suggested fix: point HOME (or the discovery's home-directory lookup) at a temporary directory in test/preload.ts, or have MCP discovery honour a test override, so the suite never reads the developer's own editor and assistant configs.