Add close/1

[mickel8]

Add a function that closes DTLS connection and invalidates the state i.e. most actions are no longer possible (e.g. performing handshake or writing data)

NOTE This PR introduces breaking changes in the API - some of the functions can now return {:ok, ...} | {:error, :closed} tuples.

Related to: elixir-webrtc/ex_webrtc#218

[Copilot]

Pull Request Overview

Adds a new close/1 function to ExDTLS that irreversibly closes a DTLS session and updates existing APIs to return {:error, :closed} on further operations.

• Delegates close/1 to the native layer with docs and a spec.
• Extends Unifex specs and C code to track a closed flag, guard operations, and implement exd_close.
• Updates existing tests to match the new return shapes and adds tests for close/1.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
lib/ex_dtls.ex	Added @doc, @spec, and defdelegate close/1 to Native.exd_close
c_src/ex_dtls/native.spec.exs	Extended specs for do_handshake, handle_timeout, write_data, handle_data, and added exd_close
c_src/ex_dtls/native.h	Introduced int closed; flag in State struct
c_src/ex_dtls/native.c	Initialized and checked closed, implemented exd_close with pending-data logic
test/retransmission_test.exs	Updated pattern match for new {:ok, ...} return from do_handshake/1
test/integration_test.exs	Updated handshake calls and added describe "disconnect" tests for close/1

Comments suppressed due to low confidence (3)

lib/ex_dtls.ex:198

• [nitpick] Expand the close/1 doc to mention that it is idempotent: subsequent calls return {:ok, []} and that other operations will return {:error, :closed}.

@doc """

test/integration_test.exs:66

• Add tests to verify that write_data/1, handle_data/1, and handle_timeout/1 on a closed session return {:error, :closed} for full coverage of the closed state.

assert {:ok, []} = ExDTLS.close(dtls)

c_src/ex_dtls/native.h:16

• [nitpick] Consider renaming the closed field to is_closed or using a proper boolean type (bool) for clarity and consistency with other flags.

int closed;