Ensure default soma voltage (v) recording for all cells

[ilkilic]

Adds a default soma voltage (v) recording for all cells if it is not already configured via reports. The recording is registered under a default_voltage report so downstream pipelines can always access membrane voltage even when no explicit voltage report is defined.

[codecov]

Codecov Report

❌ Patch coverage is 96.96970% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
tests/test_reports/test_reports_utils.py	94.44%	1 Missing ⚠️

Files with missing lines	Coverage Δ
bluecellulab/reports/utils.py	94.65% <100.00%> (+0.46%)	⬆️
tests/test_reports/test_reports_utils.py	98.33% <94.44%> (-0.47%)	⬇️

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