Fix crash in oneshot coverage when file is missing or not valid Ruby

[skryukov]

This PR fixes Coverage.line_stub raising Errno::ENOENT or SyntaxError during oneshot coverage adaptation.

Errno::ENOENT: In our case, gRPC proto stubs are auto-generated and gitignored. They exist when Ruby loads them (so Coverage tracks them), but may not exist on disk by the time SimpleCov's at_exit hook calls Coverage.line_stub. Since this happens during at_exit, it's hard to orchestrate the process. User-configured filters (add_filter) could be applied earlier in the pipeline, but BlockFilter requires SourceFile objects (which need the file on disk and parsed coverage data), so it can't work at the raw-path stage.

SyntaxError: Also fixes the issue reported in #1154 comments where Coverage.line_stub crashes on non-Ruby files (e.g. YAML) tracked via enable_coverage_for_eval.

The fix: when Coverage.line_stub fails, fall back to building the line array from the oneshot data alone. This preserves coverage results (with degraded nil/0 distinction for uncovered lines) instead of crashing.