Add mock resty.sha256 and resty.string for testing

The test environment doesn't have OpenResty libraries, so we need
to provide mock implementations for testing.

Created:
- tests/mock_resty_sha256.lua: Uses system sha256sum command to
  compute SHA-256 hashes. Mimics the resty.sha256 API (new,
  update, final).

- tests/mock_resty_string.lua: Implements to_hex() to convert
  binary strings to hexadecimal.

Updated test.lua to preload these mocks so that when the module
or tests require 'resty.sha256' or 'resty.string', they get our
mock implementations instead.

This allows the PoW verification tests to run and actually verify
the SHA-256 proof-of-work.

test.lua

@@ -2,6 +2,14 @@ lu = require('luaunit')
 local rex = require('rex_pcre2')
 require('util')
 
+-- Preload mock resty libraries for testing
+package.preload['resty.sha256'] = function()
+    return require('tests.mock_resty_sha256')
+end
+package.preload['resty.string'] = function()
+    return require('tests.mock_resty_string')
+end
+
 function createNgx()
     local ngx = {
         status = nil

tests/mock_resty_sha256.lua

@@ -0,0 +1,35 @@
+-- Mock implementation of resty.sha256 for testing
+-- Uses system sha256sum command since we don't have OpenResty libraries
+
+local sha256 = {}
+sha256.__index = sha256
+
+function sha256:new()
+    local obj = {
+        buffer = ""
+    }
+    setmetatable(obj, sha256)
+    return obj
+end
+
+function sha256:update(data)
+    self.buffer = self.buffer .. data
+end
+
+function sha256:final()
+    -- Use sha256sum command to compute hash
+    local handle = io.popen("echo -n '" .. self.buffer:gsub("'", "'\\''") .. "' | sha256sum")
+    local result = handle:read("*a")
+    handle:close()
+
+    -- Parse hex string into binary
+    local hex = result:match("^(%x+)")
+    local binary = {}
+    for i = 1, #hex, 2 do
+        table.insert(binary, string.char(tonumber(hex:sub(i, i+1), 16)))
+    end
+
+    return table.concat(binary)
+end
+
+return sha256

tests/mock_resty_string.lua

@@ -0,0 +1,13 @@
+-- Mock implementation of resty.string for testing
+
+local str = {}
+
+function str.to_hex(binary)
+    local hex = {}
+    for i = 1, #binary do
+        table.insert(hex, string.format("%02x", string.byte(binary, i)))
+    end
+    return table.concat(hex)
+end
+
+return str