fix(dns): correct error() call in dns client initialization

[suryaparua-official]

Summary

Fixes #13357

Changes

In apisix/plugins/limit-count/init.lua at line 262, the error()
function was called with incorrect arguments:

-- Before (wrong)
error("failed to generate key invalid parent: ", core.json.encode(parent))

-- After (correct)
error("failed to generate key invalid parent: " .. core.json.encode(parent))

Lua's error(message, level) expects the second argument to be an
integer (stack level: 0, 1, or 2), not a string. The previous code
would cause LuaJIT to raise:

bad argument #2 to 'error' (number expected, got string)

Test

Added a test case in t/plugin/limit-count5.t to verify the error
message is correct when _meta.parent is missing.

[shreemaan-abhishek]

we need three empty lines in between tests.

[hachi029]

A similar error occurs in apisix/discovery/dns/init.lua:81

[suryaparua-official]

Hi @hachi029, thank you for pointing that out! I will fix the similar issue in apisix/discovery/dns/init.lua:81 and include it in this PR.

[membphis]

LGTM