Billing: Implement usage-based pricing logic

[cuevaio]

Description

Implement runtime measurement and billing calculation for code executions.

Billing Rules

• Minimum 1 second billed per request
• Additional time billed in 0.5s intervals
• Always round up to nearest 0.5s

Tasks

• Implement runtime measurement (using startTime + Date.now() approach or Lambda metadata)
• Enforce minimum 1 second billing per request
• Implement 0.5s interval billing for additional time
• Implement rounding up to nearest 0.5s
• Store computed cost per execution in logs/metadata

Examples

Actual Runtime	Billed Duration
0.1s	1.0s
1.0s	1.0s
1.2s	1.5s
2.0s	2.0s
2.3s	2.5s

Acceptance Criteria

• All executions are properly measured and billed
• Billing follows the defined rounding rules
• Cost data is persisted for reporting