ipairs terminates on the first nil index when iterating over table keys:

for i,k in ipairs( {[1] = 'test', [3] = 'test'} ) do
  print(i, k)
end

prints:
1 test

Instead, use pairs which continues iterating over the entire table:

for i,k in pairs( {[1] = 'test', [3] = 'test'} ) do
  print(i, k)
end

prints:
1 test
3 test

This fixes the handler signature and also prevents n+1 requests firing
if there are multiple clients.

(The first `prepareCallHierarchy` handler is called once per client,
each invocation used `buf_request` to make more requests using *all*
clients)

Given that the input is pre-filled with a path, it should be possible to
use dir completion.

Add the buffer number to the `textDocument/formatting` request, so
that it is passed to the handler.
The built-in formatting handlers do not use the buffer number, but user
overrides should have access to it.