Fix audio cost per second override - #19158

Relevant issues

Fixes #19154

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CI (LiteLLM team)

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• 50-55 passing tests: main is stable with minor issues.
• 45-49 passing tests: acceptable but needs attention
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Type

🐛 Bug Fix
📖 Documentation
✅ Test

Changes

• Fix audio cost-per-second logic so output_cost_per_second only applies when > 0, and input_cost_per_second still bills even if output is 0.
• Add a regression test for transcription models configured with input_cost_per_second and output_cost_per_second: 0.
• Clarify custom pricing docs for cost-per-second behavior and zero output cost.