feat(lsp)!: vim.lsp.inlay_hint.get(), enable(), is_enabled() #25512

refactor!: `vim.lsp.inlay_hint()` -> `vim.lsp.inlay_hint.enable()`

Problem:
The LSP specification allows inlay hints to include tooltips, clickable
label parts, and code actions; but Neovim provides no API to query for
these.

Solution:
Add minimal viable extension point from which plugins can query for
inlay hints in a range, in order to build functionality on top of.

Possible Next Steps
---

- Add `virt_text_idx` field to `vim.fn.getmousepos()` return value, for
  usage in mappings of `<LeftMouse>`, `<C-LeftMouse>`, etc

test/functional/helpers.lua

@@ -18,8 +18,7 @@ local sleep = global_helpers.sleep
 local tbl_contains = global_helpers.tbl_contains
 local fail = global_helpers.fail
 
-local module = {
-}
+local module = {}
 
 local start_dir = luv.cwd()
 local runtime_set = 'set runtimepath^=./build/lib/nvim/'
@@ -834,6 +833,8 @@ function module.exec_capture(code)
   return module.meths.exec2(code, { output = true }).output
 end
 
+--- @param code string
+--- @return any
 function module.exec_lua(code, ...)
   return module.meths.exec_lua(code, {...})
 end
@@ -948,8 +949,10 @@ function module.mkdir_p(path)
     or 'mkdir -p '..path))
 end
 
+--- @class test.functional.helpers: test.helpers
 module = global_helpers.tbl_extend('error', module, global_helpers)
 
+--- @return test.functional.helpers
 return function(after_each)
   if after_each then
     after_each(function()