Add regression tests for backtrace source mapping

[JPDuchesne]

Summary

• Answers "is the Minitest plugin backtrace filter still necessary after this year's source-mapping fixes?" — yes. ast-transform's e65a92c (compile ISeq with the original source path) made backtrace file paths correct without the filter, but line numbers still come from the transformed source; only RSpock::BacktraceFilter + the SourceMap maps them back. None of this was covered by tests.
• test/rspock/backtrace_filter_test.rb: unit tests for filter_string (line mapping, transformed-path mapping, ? for unmappable lines, pass-through without a SourceMap), filter_exception, and the RSpock::Minitest::BacktraceFilter wrapper — all against a real SourceMap registered by running a fixture through the actual transform pipeline in a tmpdir (no filesystem mocks).
• test/rspock/backtrace_source_mapping_test.rb: subprocess integration test running a genuinely failing RSpock test file end to end, asserting failure output cites source line numbers (computed from the fixture, not hardcoded). A second test runs the same fixture with the plugin filter left uninstalled and asserts the cited lines differ — this is the canary that fails if the plugin ever genuinely becomes redundant, so future cleanup can be evidence-based.

Notes for review

• Fixture sources deliberately use blank lines and multi-line expressions so source and transformed line numbers diverge — a broken mapping cannot pass by coincidence.
• The child process runs with MT_NO_PLUGINS=1 (minitest's gem-scan plugin discovery activates the Ruby default-gem minitest over the -I one and breaks the run); the enabled variant calls Minitest.plugin_rspock_init({}) explicitly — the same call plugin discovery would make.
• macOS detail: tmpdir paths are realpath'd because backtrace_locations resolves /var symlinks to /private/var, and SourceMap registration is keyed by exact path string.

Test plan

• bundle exec rake test — 236 tests, 0 failures (was 228)
• Both new files individually green

Made with Cursor

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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