Improve python CodeIntelligence via using the deepnote_toolkit LSP

[saltenasl]

We currently have rudimentary code intelligence in python blocks. deepnote_toolkit exposes a LSP server that is readily available. We need to hook up the existing kernelCompletionProvider with it. It should enable:

1. Code completion
2. Hover documentation
3. Go to definition
4. Symbol information

[coderabbitai]

A summary of the changes CodeRabbit can apply:

• Integrate Deepnote's deepnote_toolkit LSP into notebook intellisense by adding new LSP client/manager/hover/definition files (src/standalone/intellisense/lsp/deepnoteLspClient.ts, src/standalone/intellisense/lsp/lspClientManager.ts, src/standalone/intellisense/deepnoteHoverProvider.ts, src/standalone/intellisense/deepnoteDefinitionProvider.ts), update kernelCompletionProvider.ts to detect Deepnote kernels and prefer LSP-based completions with kernel fallback, and register the new providers in src/standalone/intellisense/serviceRegistry.node.ts.

• Integrate the deepnote_toolkit LSP into the notebook completion flow by adding a reusable LSP client and manager, creating hover/definition/symbols providers, and wiring them into the service registry — modify src/kernels/helpers.ts (add isDeepnoteKernelConnection), update src/standalone/intellisense/kernelCompletionProvider.ts (initialize/use LSP client and fallback to kernel), update src/standalone/intellisense/serviceRegistry.node.ts (register providers), and add new files src/standalone/intellisense/lsp/deepnoteLspClient.ts, src/standalone/intellisense/lsp/lspClientManager.ts, src/standalone/intellisense/deepnoteHoverProvider.ts, src/standalone/intellisense/deepnoteDefinitionProvider.ts, and src/standalone/intellisense/deepnoteSymbolsProvider.ts.

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