feat: Add ER diagram visualization for database schemas

[jcjaskula-aws]

ER Diagram Visualization for Database Schemas

This PR adds an interactive Entity-Relationship diagram feature to SQLTools. Right-click any schema node in the connection explorer and select "Show ER Diagram" to visualize tables, columns, and foreign key relationships.

Features

• Interactive diagram built with React Flow — pan, zoom, and drag tables to rearrange
• Column-level FK connections — relationship lines connect from the specific FK column to the referenced PK column
• Dagre auto-layout — tables are positioned using a directed graph layout algorithm that minimizes edge crossings
• Per-table color coding — each table gets a consistent color; its outgoing FK edges use the same color for easy tracing
• Focus mode — click a table (or use the search field in the toolbar) to highlight it and its connected tables, dimming everything else. Press Escape or click the background to clear
• Searchable table picker — type to filter tables in the toolbar, select to focus and pan to the table
• VS Code theme integration — uses CSS variables so it adapts to any light or dark theme
• Multi-driver support — foreign key queries implemented for PostgreSQL, MySQL, MSSQL, and SQLite

Bug fix

• Fixed a pre-existing event listener leak in the PG driver where notice handlers accumulated on pooled clients across queries

Changes across packages

• types — Added NSDatabase.IForeignKey interface, fetchForeignKeys query builder, getERDiagramData driver method
• base-driver — Implemented getERDiagramData in AbstractDriver
• driver.pg/mysql/mssql/sqlite — Added fetchForeignKeys SQL queries
• driver.pg — Fixed notice listener leak in query method
• language-server — Added GetERDiagramDataRequest handler, Connection.getERDiagramData delegation
• plugins/connection-manager — New command, LSP contract, webview provider, and React Flow UI
• extension — Registered command, context menu on schema nodes, webpack entry point for ERDiagram webview

How to test

1. Connect to a PostgreSQL (or MySQL/MSSQL/SQLite) database
2. Expand the connection → database → schemas in the sidebar
3. Right-click a schema → "Show ER Diagram"
4. Verify tables render with columns, PK/FK badges, and colored relationship lines
5. Click a table to enter focus mode; use the search field to find tables
6. Drag tables, pan, and zoom to verify interactivity

Notes

I meant to write something that would solve #1304 but it is still lacking editing. Totally fine if a solution based on erd-editor is preferred.

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