Reject failed input streams

[henriquejsza]

Fixes #408

• reject failed input streams before and during parsing
• preserve empty and EOF-only streams
• add regression coverage for Load and LoadAll

Tests:

• CMake C++11 static build and full test suite
• Clang C++23 shared build and full CTest
• GCC C++20 ASan/UBSan suite
• Bazel build :all and test test

[henriquejsza]

CI follow-up: the six Windows x64 MinGW jobs built successfully but hung in CTest when launched under Git Bash. In that shell, /mingw64/bin precedes the compiler-matching C:\mingw64\bin; prepending /c/mingw64/bin only for the affected matrix entries makes the full suite complete. This was verified in diagnostic run 32585105051, where the job passed in 1m40s. Commit f1c2b7f applies only that PATH adjustment; the refreshed upstream workflow is currently awaiting maintainer approval.

[SGSSGene]

Thank for this fix! Especially finding the issue in the CI.

Short remark: This PR now contains three changes:

• fixing bad input streams
• fixing CI
• and cleanup raw pointers -> std::unique_ptr

I would prefer to have the raw pointers -> std::unique_ptr in a separately PR. It makes following the changes easier.

But again! This fixes are appreciated!