Add regression assertion for notebook cell.source format consistency

[Copilot]

The regression suite compared cell content via normalized strings, which could hide mixed cell.source representations (string vs string[]) in resolved notebooks. This PR adds a targeted post-resolution check to ensure source representation is uniform across all cells.

• What changed

  • Added assertConsistentCellSourceFormat(notebook) in src/tests/testHelpers.ts.
  • The helper infers expected source shape from the first cell and fails if any subsequent cell differs.
  • Integrated this assertion into src/tests/incomingNonConflictRegression.test.ts immediately after notebook-structure validation.

• Why this matters

  • Preserves visibility into source-shape inconsistencies that content-only normalization can mask.
  • Strengthens regression coverage for notebook-tool compatibility expectations around cell.source formatting.

• Example

  // src/tests/incomingNonConflictRegression.test.ts
  const resolvedNotebook = await applyResolutionAndReadNotebook(page, session.conflictFile);
  validateNotebookStructure(resolvedNotebook);
  assertConsistentCellSourceFormat(resolvedNotebook);

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Original prompt

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This section details on the original issue you should resolve

<issue_title>[Test] Verify source format consistency (string vs array)</issue_title>
<issue_description>## Context
From PR #34 review discussion: #34

The integration test normalizes all cell sources to strings for comparison, which could mask format inconsistencies where some cells have source as a string and others as an array.

Problem

• Resolved conflict cells use: source: resolvedContent.split(/(?<=\n)/) (always array)
• Identical cells use: cellToUse = currentCell || incomingCell || baseCell (keeps original format)
• Test's getCellSource() normalizes both, hiding potential issues

Impact

Low priority - doesn't affect core functionality but could cause issues with some notebook tools that expect consistent formatting.

Suggested Enhancement

Add a check after writing the notebook to verify all cells use the same source format (all strings or all arrays).

Backlinks:

• PR [FEAT] Create a Testing Harness #34: [FEAT] Create a Testing Harness #34
• Requested by: @Avni2000</issue_description>

Comments on the Issue (you are @copilot in this section)

@Avni2000 @coderabbitai is this still an issue?

• Fixes [Test] Verify source format consistency (string vs array) #35

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