Invalid JSON log does not contain initDurationMs/restoreDurationMs

## Description

When a Lambda function has its log format set to **JSON** (via AWS Advanced Logging Controls), the `executor` Lambda fails with:

```
Error: Invalid JSON log does not contain initDurationMs
```

or

```
Error: Invalid JSON log does not contain restoreDurationMs
```

or

```
Error: Invalid JSON log does not contain billedRestoreDurationMs
```

## Root Cause

After a quickly investigation the problem, apparently, is in `lambda/utils.js`, the function `extractDurationFromJSON` performs a strict string check:

```javascript
if (!log.includes(durationType)) {
throw new Error(`Invalid JSON log does not contain ${durationType}`);
}
```

This throws an exception when `initDurationMs` (or `restoreDurationMs` or `billedRestoreDurationMs`) is absent from the log:

- initDurationMs — absent on warm start invocations (only present on cold starts)
- restoreDurationMs — absent on non-SnapStart Lambda functions
- billedRestoreDurationMs — absent on non-SnapStart Lambda functions

By contrast, the **text** log path (`extractDurationFromText`) handles this gracefully by returning `0` when the regex doesn't match:

```javascript
if (match == null) return 0;
```

The JSON path should behave the same way — return `0` instead of throwing.

## Steps to Reproduce

1. Deploy a Lambda function with log format set to **JSON** (Monitoring and operations tools → Logging configuration → Log format → JSON)
2. Run the Power Tuning state machine targeting that function
3. The `executor` step fails on the second or subsequent invocations (warm starts) because `initDurationMs` is not present in the log

## Expected Behavior

The `extractDurationFromJSON` and `extractDurationFromText` functions should work in the same way giving the same outputs.
The tool should work and complete the happy flow without any exception.

## Workaround

Temporarily switch the Lambda's log format from JSON to Text before running the tuning, then switch it back.

## Environment

- Power Tuning version: latest (deployed via SAR on 2026-04-22)
- Region: eu-central-1 (tool) / eu-south-1 (target Lambda)
- Lambda log format: JSON (AWS Advanced Logging Controls)