Sort endpoints in validation stats for deterministic output

[Copilot]

endpoints_used was built from a set and converted directly to list, producing nondeterministic ordering that made stats output unstable across runs.

Changes

• Sort endpoints alphabetically before storing in stats["endpoints_used"] (line 171)
• Sort endpoints in mixed endpoint warning message for consistency (line 183)

# Before
result.stats["endpoints_used"] = list(endpoints)

# After  
result.stats["endpoints_used"] = sorted(endpoints)

This ensures stable output for testing and comparison while maintaining the same API.

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[Copilot]

Pull request overview

This PR makes validation statistics and warning output deterministic by sorting endpoint lists derived from a set, avoiding run-to-run ordering instability.

Changes:

• Sort endpoints_used before storing it in result.stats for stable stats output.
• Sort the endpoint list in the “mixed endpoints” warning message for consistent messaging.

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