Function format_diagnostics_text 

pub fn format_diagnostics_text(
    report: &DiagnosticsReport,
    filtered_count: usize,
) -> String

Format diagnostics report for compact, token-efficient text output

R1: One-line summary header, one-line-per-diagnostic, no decorations, no ANSI colors R2: Strips absolute paths to relative using common_path_prefix R3: Truncates multi-line messages (pyright nested explanations) to first line