Unify resource extraction methods in Duo Chat executors

What does this MR do and why?

Remove duplicated extract_resource methods from commit, epic, issue, and merge request readers and move them to the parent ee/lib/gitlab/llm/chain/tools/identifier.rb class.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Run Duo Chat.
2. Enter in the chat a question with a url:
   • Summarize this issue: http://127.0.0.1:3000//gitlab-org/gitlab-test/-/issues/8
   • Summarize this epic: http://127.0.0.1:3000/groups/gitlab-org/-/epics/11
   • Summarize this MR: http://127.0.0.1:3000/gitlab-org/gitlab-test/-/merge_requests/9

Related to #480261 (closed)