fix: propagate errgroup context to lifecycle phases

[Pranav-IIITM]

Summary

Propagate the parent build context through the errgroup used for concurrent build and run-extension phases.

This ensures that:

• User cancellation reaches all running phases.
• A failure in one phase cancels sibling phases.
• The concurrent phases no longer use an unrelated context.TODO().

Output

Before

group, _ := errgroup.WithContext(context.TODO())
return l.Build(ctx, phaseFactory)

The errgroup context was discarded, and the parent context was not used.

After

group, groupCtx := errgroup.WithContext(ctx)
return l.Build(groupCtx, phaseFactory)

The derived context is now passed to Build, ExtendBuild, and ExtendRun.

Documentation

• Should this change be documented?
  • Yes, see #___
  • No

This is an internal lifecycle execution and cancellation fix with no user-facing configuration or CLI changes.

Testing

• git diff --check passed.
• Phase tests passed: 18 passed, 3 skipped.
• One Windows Docker-backed test remains environment-specific: the symlink assertion reports the copied symlink as a regular file. This is unrelated to the changed code.

Related

closes #2700