Execute selection, from cursor, and current batch (#53)

[erikdarlingdata]

Summary

• Execute highlighted text: F5/Ctrl+E/Ctrl+L now run only the selected text when a selection exists
• Execute from Cursor: right-click context menu, runs from cursor position to end of editor
• Execute Current Batch: right-click context menu, runs the batch between GO separators where the cursor sits
• Multi-statement plan fix: both estimated and actual plan executors now collect all result sets and merge ShowPlanXML documents, so all statements appear in the Statements panel

Closes #53

Test plan

• Two statements without GO — Ctrl+E shows both in Statements panel
• Highlight first statement only — Ctrl+E shows only that plan
• Right-click → Execute from Cursor at first statement — shows both
• Two statements separated by GO — right-click → Execute Current Batch on first shows only first
• Cancel during multi-statement execution removes the tab

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