feat(lsp): add documentSymbol, signatureHelp, and codeAction methods

- Add textDocument/documentSymbol for SQL statement outline
- Add textDocument/signatureHelp with signatures for 20+ SQL functions
- Add textDocument/codeAction for quick fixes (semicolon, uppercase)
- Add protocol types: DocumentSymbol, SignatureHelp, CodeAction, etc.
- Update ServerCapabilities to advertise new features

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: 2 people