Fix documentation retrieval for callbacks

[icostan]

Current mix usage_rules.docs does not return doc for callback, it only works for module/function using the IEx.Helpers.h/1 function.

usage_rules % mix usage_rules.docs GenServer.handle_call                                             [0]
Compiling 1 file (.ex)
Generated usage_rules app
Searching local docs for 

    GenServer.handle_call

Use `mix usage_rules.search_docs` to search online documentation for more results.


No documentation for function GenServer.handle_call was found, but there is a callback with the same name.
You can view callback documentation with the b/1 helper.

This PR:

• Improve callback detection in usage_rules.docs task using robust regex
• Add comprehensive tests for the docs task covering modules, functions, and callbacks

usage_rules % mix usage_rules.docs GenServer.handle_call                                             [3]
Searching local docs for 

    GenServer.handle_call

Use `mix usage_rules.search_docs` to search online documentation for more results.


@callback handle_call(request :: term(), from(), state :: term()) ::
            {:reply, reply, new_state}
            | {:reply, reply, new_state,
               timeout() | :hibernate | {:continue, continue_arg :: term()}}
            | {:noreply, new_state}
            | {:noreply, new_state,
               timeout() | :hibernate | {:continue, continue_arg :: term()}}
            | {:stop, reason, reply, new_state}
            | {:stop, reason, new_state}
          when reply: term(), new_state: term(), reason: term()

Invoked to handle synchronous call/3 messages. call/3 will block until a reply
is received (unless the call times out or nodes are disconnected).

request is the request message sent by a call/3, from is a 2-tuple containing
the caller's PID and a term that uniquely identifies the call, and state is the
current state of the GenServer.
...

Contributor checklist

Leave anything that you believe does not apply unchecked.

• I accept the AI Policy, or AI was not used in the creation of this PR.
• Bug fixes include regression tests
• Chores
• Documentation changes
• Features include unit/acceptance tests
• Refactoring
• Update dependencies

[zachdaniel]

Looks like tests failing/format failing.