test(core): cover the variadic form of response.exit and response.say

[AlinsRan]

Description

resp_exit is variadic — it accepts a non-numeric first argument (no status code), and any number of body parts which are printed in order, with tables encoded as JSON:

local function resp_exit(code, ...)
    ...
    if code and type(code) ~= "number" then
        idx = idx + 1
        t[idx] = code
        code = nil
    end

t/core/response.t only ever calls exit(code, body), so none of that is covered, and _M.say is not exercised at all. apisix/plugins/mcp/server_wrapper.lua already depends on the variadic form:

return core.response.exit(message_handler(conf, ctx, opts))

Changes

Three tests, no production code touched:

• TEST 9 — exit("done\n"): no status code, body still written, response stays 200
• TEST 10 — exit(201, "a", {b = "b"}, "c\n"): parts printed in order, the table encoded as JSON
• TEST 11 — say("a", "b", "c\n")

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation accordingly
• I have verified that the change is backward compatible