Issue #3586276: Rename entity bundle and target entity scope plugins for better DX

Description

Follow-up to #3586148 (closed). The Entity Bundles and Target Entities scopes had near-identical descriptions ("… relevant for this context"), so users couldn't tell the type-level scope from the instance-level one. This relabels them and rewrites the help text to make the distinction clear, using the exact copy from the UX review.

Testing instructions

• Updated the affected scope tests (label / manage-label / page-title assertions).
• Relevant Unit, Kernel and Functional tests pass; phpcs (Drupal + DrupalPractice) clean.

Manual testing checklist

1. Go to: /admin/config/ai/context/settings/scope Do: Look at the scope list Expect: Rows read Entity Types and Specific Entities (no "Entity Bundles"/"Target Entities")
2. Go to: /admin/config/ai/context/items/add Do: Open the Context Scope section Expect: Entity Types shows "Apply this context to every entity of a chosen type…"; Specific Entities shows "Apply this context only to the entities chosen below…" — the two now read as clearly different
3. Go to: same form, under Specific Entities Do: Look at the entity picker Expect: The autocomplete appears with no "Target entities" heading above it (that's the field-label removal)
4. Go to: same form Do: Click Manage Entity Types / Manage Specific Entities links. Expect: They open the two settings pages (titles "Entity Types Settings" / "Specific Entities Settings")
5. Go to: /admin/config/ai/context/settings/agents Do: edit any agent, scroll the scope subscriptions. Expect: Entity types is offered as a subscribable scope; Specific Entities is not.

• I have updated the MR title to use format: Issue #1234: My issue title.
• I have noted any required post-merge steps (config imports, cache rebuilds, manual changes)
• I have updated the MR description to include: Closes #1234
• I have performed a self-review of my own code
• This MR contains no breaking API or hook changes, or they are explicitly documented in the description
• I have added or updated tests, or explained in the description why this change is not covered by tests
• I have updated documentation for any new or changed functionality
• I have written testing instructions and verified them locally

AI Compliance

Note

• AI Assisted Code
  Mainly written by a human; AI used for autocomplete or partial generation under full human supervision.
• AI Generated Code
  Mainly generated by AI, reviewed and approved by a human before this MR was created.
• Vibe Coded
  Generated by AI and only functionally reviewed before this MR was created.

Closes #3586276.