Add test coverage for runScripts orchestration in script provisioning provider

## Description

Add test coverage for `runScripts` in `extensions/microsoft.azd.scripts/internal/provisioning/provider.go`. This is the main orchestration function that coordinates script execution, error handling, output merging, and progress reporting.

## Context

Identified during PR #7737 review by @jongio. Individual components (config parsing, env resolution, executor, output collector) all have unit tests, but the orchestrator function lacks coverage.

## Test Cases Needed

- Script execution order and progress callbacks
- Output merging across multiple scripts
- `ContinueOnError` behavior (continue vs stop)
- Context cancellation propagation

## Related

- PR #7737 (Script Provisioning Provider)