Function build_context 

pub fn build_context(
    request: &ContextRequest,
    source: &str,
    diagnostics: &[Diagnostic],
    provenance: Option<&Provenance>,
) -> ContextBundle

Assemble a bundle from already-read source and optional trace results.

This is the pure core: no filesystem access and no process spawning, so it is the right entry point for tests and for callers that already hold the source text and diagnostics.