The OPTIONS preflight hardcoded Access-Control-Allow-Headers to just
Content-Type. Any browser MCP client sending mcp-protocol-version (per
the MCP spec) or Authorization / X-API-Key (typical for hosted
deployments) failed the preflight and the browser blocked the actual
request.
Add those three to the allow-list and a 24h Max-Age so browsers don't
re-preflight every call. Adds an integration test that drives the
server with a real preflight request.
Closes#175.
SGavrl