Add LSP diagnostic test files for multiple languages

Includes test files for Lua, JavaScript, PHP to verify the Paper Tonic
Modern colorscheme's diagnostic virtual text colors across different
LSPs and their expected diagnostics.

test-diagnostics.lua

@@ -0,0 +1,84 @@
+-- Test file for LSP diagnostics - lua_ls
+-- This file intentionally contains different diagnostic levels
+-- to test diagnostic virtual text colors in paper-tonic-modern
+
+-- Define some valid code first
+local valid_function = function()
+  return "valid"
+end
+
+---@diagnostic disable-next-line: unused-local
+local _ = valid_function()
+
+-- ERROR: Undefined global (lua_ls marks this as error)
+print(undefined_global_variable)
+
+-- ERROR: Undefined function
+undefined_function()
+
+-- WARNING: Unused local variable (lua_ls warning level)
+local unused_variable = "I'm never used"
+
+-- WARNING: Lowercase global (should be local)
+lowercase_global = "This should be local or uppercase"
+
+-- HINT: Redundant parentheses (some configs treat as hint)
+local x = (5)
+print(x)
+
+-- ERROR: Wrong number of arguments
+local function takes_two_args(a, b)
+  return a + b
+end
+takes_two_args(1)  -- Missing second argument (lua_ls error)
+
+-- ERROR: Attempting to call a non-function
+local not_a_function = "string"
+not_a_function()
+
+-- WARNING: Shadowing outer local
+local shadow_me = 1
+do
+  local shadow_me = 2  -- Shadows outer local (warning)
+  print(shadow_me)
+end
+
+-- ERROR: Invalid table access
+local empty_table = {}
+print(empty_table.nonexistent.deeply.nested.property)
+
+-- WARNING/HINT: Can be simplified
+if true == true then  -- Redundant comparison
+  print("always true")
+end
+
+-- ERROR: Type annotation mismatch
+---@type string
+local should_be_string = 123  -- Assigning number to string type
+
+-- WARNING: Unreachable code after return
+local function has_dead_code()
+  return "done"
+  print("This will never execute")  -- Unreachable
+end
+
+-- Valid function call
+print(takes_two_args(1, 2))
+
+-- ERROR: Calling vim API that doesn't exist
+vim.nonexistent_function()
+
+-- HINT: Trailing whitespace or style issues (if configured)
+local with_trailing_space = "value"   
+print(with_trailing_space)
+
+-- ERROR: Parameter type mismatch
+---@param name string
+---@param age number
+local function greet(name, age)
+  print("Hello, " .. name .. ", you are " .. age)
+end
+greet(123, "not a number")  -- Wrong types
+
+print("Diagnostic test complete!")
+