fix(sql-editor): remove slash-triggered AI entry

[openai0229]

Related issue

Related to #2255

Summary

Remove the legacy /-triggered AI entry from the SQL editor because the current interaction is unreliable and conflicts with normal SQL input.

This removes both BuiltInCopilot implementations, Monaco overlay/view-zone handling, slash and global keyboard listeners, the slash-trigger placeholder, the legacy AI float layer, and workspace state used only by that flow. Explicit AI features such as the right-side AI panel, SQL optimization, and AI table creation remain unchanged.

The replacement interaction and execution model will be designed separately in Discussion #2256.

Affected surfaces

• Frontend / Web
• Backend / API / Storage
• Database plugin / Driver
• JCEF / Desktop packaging
• CI / Build / Release
• Documentation only

Verification

• Commands and results:
  • yarn lint - passed
  • yarn test:i18n - passed
  • yarn build:web:community --app_version=5.3.0 - passed
  • git diff --check - passed
• Manual verification: Community backend http://localhost:10825/api/system, frontend http://localhost:8889/workspace, and the frontend-proxied API returned HTTP 200 after restart.
• UI evidence: N/A - browser automation was unavailable; the Community Web production build and runtime health checks passed.

Risk and compatibility

• Public API or stored data: N/A - no API or persistence changes.
• Database or driver compatibility: N/A - no database or driver changes.
• Network, privacy, or security: N/A - no network, privacy, or security behavior changes.
• Community / Local / Pro boundary: Community frontend only. Explicit AI entry points remain available.
• Backward compatibility: Removes only the / keyboard trigger and its dedicated overlay/state path; normal SQL editing is preserved.

Reviewer map

• Start here: chat2db-community-client/src/components/SQLEditor/editor/SQLEditorWithOperation/index.tsx
• Failure condition: Typing / still opens an AI overlay, or normal SQL editing and explicit AI features regress.
• Rollback or disable path: Revert this commit to restore the removed legacy trigger path.

Contributor declaration

• I linked the Issue that defines this change.
• I tested the affected behavior and reported the actual results above.
• I did not include credentials, private data, or generated build output.
• I disclosed substantial AI assistance below, or this PR contains no substantial AI-generated code.

AI assistance: Substantial AI assistance was used for implementation, cleanup, and verification.