Issue #3586303: Announce filtered row counts to screen readers in JS filter behaviors.

Description

• Added core/drupal.announce as a library dependency in ai_context.libraries.yml so Drupal.announce() is guaranteed available.
• js/ai_context_items_filter.js: after every filter change (apply()), counts visible tbody rows and calls Drupal.announce() with a pluralized translated message — e.g. "3 context items shown." or "No context items match the current filter."
• js/ai_context_agents.js: after every search input event, counts visible rows and calls Drupal.announce() — e.g. "2 agents shown." or "No agents match the current filter."

4.2 AJAX reattachment

Verified — neither the context items table nor the agents overview form is replaced via AJAX in the current UI. The once() guards are correctly placed on the top-level elements. No fix needed.

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

AI Compliance

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Closes #3586303