fix(test): preserve full stack traces in test.step with POM and Promise.all

[ishanrajsingh]

Issue #40141

Problem

Stack traces are truncated when using test.step inside Page Object Model (POM) methods,
especially when combined with Promise.all.

User-land stack frames (e.g. POM methods and test file calls) were missing,
making debugging difficult.

Root Cause

Stack traces were being overwritten in intermediate step handling:

• _addStep.complete
• _failWithError

This removed original async stack frames.

Fix

• Preserve original error stack
• Append step context only in _failWithError (final error pipeline)
• Avoid modifying stack in _addStep.complete
• Improve stack capture and parsing in stackTrace.ts

Result

Stack traces now correctly include:

• POM methods (e.g. shouldBeCorrect, shouldBeValidColor)
• test file call site
• step hierarchy (test.step)

JavaScript async limitations (e.g. Promise.all) may still omit some parent frames,
but Playwright no longer truncates or removes user-land stack traces.

Validation

• Verified via CLI output
• Verified via Trace Viewer step hierarchy
• Reproduced using minimal test case from issue