feat: add TelegramEx.Error struct for API errors

[SAY-5]

Description

Adds a TelegramEx.Error struct so API errors surface their code, description, and retry hint instead of collapsing to a single :bad_request atom. TelegramEx.API now returns {:error, %TelegramEx.Error{}} from both error-handling paths (get_updates/2 and the shared handle_response/1), parsing error_code, description, and parameters.retry_after from the response body and keeping the raw payload around. This makes it possible to, for example, read retry_after on a 429 and back off appropriately.

Ready Checklist

• Implementation: code works, types are specified, dialyzer passes
• Documentation: functions have @doc, README updated if needed
• Tests: tests added/updated, mix test passes locally
• Example: usage example added to /example

Related Issues

Closes #19

[lsdrfrx]

Thanks for your contribution! Truly appreciated. 👏