Add CSV Export for Test Results

[aviralgarg05]

Description

SuiteResult in src/agentunit/reporting/results.py supports JSON, Markdown, JUnit, and HTML exports.
It lacks CSV export, which is often requested for analyzing results in spreadsheets.

Task

• Add a to_csv(self, path: str | Path) method to SuiteResult.
• Flatten nested metrics into columns (e.g., metric_ExactMatch, metric_Latency).

Source

https://github.com/aviralgarg05/agentunit/blob/main/src/agentunit/reporting/results.py

[shihabhasan0161]

@aviralgarg05 Can you assign this to me?