Data Connections: Add positronDataConnections.getSchema command

[timtmok]

Context

Part of the implementation plan in #14658 for cold-start Assistant awareness of database schema. #14923 (positronDataConnections.getConnections) already exposes connection/profile metadata but deliberately excludes table/column schema ("rich schema context is #14658"). This command closes that gap. This is issue 2 of 2; depends on the schema-summarization helper added in #14926.

Consumption of this command from Posit Assistant (curated command list entry, skill guidance, approval safelist) is in the scope of #14693's executeCommand tool, not this issue.

AC

• Add positronDataConnections.getSchema to positronDataConnectionsCommands.ts (same file as getConnections/connectInConsole from Data Connections: Add positronDataConnections.getConnections command #14923/Data Connections: Add positronDataConnections.connectInConsole command #14924), registered from the contribution file.
• Args: { profileId: string; maxDepth?: number }. Resolves the live instance via IPositronDataConnectionsService.getInstanceForProfile(profileId), calls summarizeDataConnectionSchema() on its connectionHandle.
• If no live instance exists for the profile, return { connected: false } rather than throwing - mirrors how getConnections reports disconnected profiles.
• Always registered; when dataConnections.enabled is off, returns { enabled: false }, matching Data Connections: Add positronDataConnections.getConnections command #14923's always-registered pattern - no ai.enabled check.

Testing

• Vitest: connected profile returns real schema; disconnected profile returns { connected: false }; feature flag off returns { enabled: false }; large schema triggers truncation markers; payload survives JSON.parse(JSON.stringify(...)) deep-equal.
• Manual verification: from the extension host / debug console (same manual route SPIKE - Data Connections: Explore how Assistant will execute R/Python queries with awareness of preferred package #14664 uses pre-Give assistant access to Positron commands #14693), call vscode.commands.executeCommand('positronDataConnections.getSchema', { profileId }) against a live SQLite/DuckDB connection and confirm real table/column names/types.