Issue #3586376: Add context item priority field and selection tie-breakers.

Description

Adds a Priority field (High / Medium / Low) on context items and uses it for deterministic selection ordering instead of the changed timestamp.

What changed

• AiContextItemPriority — constants for High (10), Medium (5), Low (0); Medium is the default.

• priority base field on ai_context_item, with getPriority() on the entity.

• Context item form — Priority control in Context Scope (above Global); hidden on the default form display config.

• ai_context_update_10015() — installs field storage and removes priority from the default form display on existing sites.

• AiContextSelector — shared compareSelectionPreference() ordering:

  1. Priority descending
  2. Shorter content length
  3. Higher item ID (final tie-break)

  Applied within each group:

  • Global — bounded top-K via insertTopPreferenceCandidate()
  • Always-include — sorted by priority; lower-priority items drop first when capped
  • Auto-included — same bounded preference helper (replaces chunk scan order)
  • Scored — priority breaks score ties
  • Match-all — sorted by priority

  Between-group order is unchanged: global → always-include → auto-included → scored.

• Docs — context_items.md, context_selection.md, and scopes.md updated for Priority and tie-breaking behavior.

• Tests — kernel selector tests updated/added; functional form test asserts the Priority field on add.

Machine name is priority (not ranking) to avoid confusion with subscription scope scoring.

Closes #3586376

Testing instructions

Automated

cd web/modules/contrib/ai_context
./lint.sh
ddev exec vendor/bin/phpunit web/modules/contrib/ai_context/tests/src/Kernel/AiContextSelectorAlgorithmTest.php
ddev exec vendor/bin/phpunit --filter testContextItemAddFormShowsPriorityField web/modules/contrib/ai_context/tests/src/Functional/AiContextScopeContextItemFormTest.php

Manual

1. Run ddev exec drush updatedb -y && ddev exec drush cr on an existing site (applies update 10015).
2. Add or edit a context item at /admin/config/ai/context/items/add — confirm Priority appears in Context Scope above Global.
3. Create several global items with different priorities and run selection with a low max_global_items — higher-priority items should appear first; lowest should be dropped.
4. Add several items to an agent Always include list (mixed request order) with different priorities and a low max_items — confirm highest-priority items are kept and ordered first.
5. With Dynamic Entity Reference enabled, create target-entity auto-included items with different priorities on the same entity context — confirm higher priority is ordered first and survives caps.

Post-merge steps

• Run database updates on existing sites: drush updatedb
• Cache rebuild: drush cr

No config import required beyond what updatedb applies.

Checklist

• I have updated the MR title to use format: Issue #1234: My issue title.
• I have updated the MR description to include: Closes #1234
• I have performed a self-review of my own code
• 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
• I have noted any required post-merge steps (config imports, cache rebuilds, manual changes)
• This MR contains no breaking API or hook changes, or they are explicitly documented in the description

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