fix(run): preserve script stdout when an Upload-and-Run file fails

[rugpanov]

Problem

When running a Python file on a cluster ("Upload and Run File"), if the script raises an exception, any output it printed before failing is lost. The 1.2/commands execution API returns only the traceback on an error result and drops the accumulated stdout, so the output panel shows the traceback with none of the preceding print output — making failures hard to debug.

Fix

Because a single command yields a single result whose error variant has no stdout field, the captured output has to be preserved where it still exists — in the bootstrap that runs the file.

• resources/python/bootstrap.py buffers the script's stdout while it runs. On an exception that follows printed output, it emits the captured stdout, a boundary marker, then a structured JSON traceback containing only the user's frames. Errors with no prior output re-raise unchanged; successful runs and SystemExit/KeyboardInterrupt replay the buffer, so no output is lost.
• src/run/ErrorParser.ts adds renderErrorEnvelope() to turn that JSON payload into renderable frames (red-highlighted summary line), plus a shared boundary constant.
• src/run/DatabricksRuntime.ts splits the text result on the boundary — emitting the stdout first, then the traceback remapped to local files (so frames stay clickable) — and shares the frame-emit logic between the new path and the existing error path.

The new behaviour triggers only on print-then-crash. Successful runs and errors with no prior output render exactly as before.

Verification

• yarn build (full tsc) clean; eslint and prettier clean; bootstrap.py compiles.
• Unit tests 4/4 pass (2 new for renderErrorEnvelope).
• End-to-end against a live cluster running the real bootstrap.py:
  • print-then-crash → stdout preserved, followed by a clickable traceback with only the user frame;
  • success + output → plain text output, unchanged;
  • error with no prior output → native error result, unchanged.

This pull request and its description were written by Isaac.

[rugpanov]

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Inputs:

• PR number: 2171
• Commit SHA: 4b846d4b52cdd591a9576059b7fadf417541a5d1

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