fix: prevent SU2_PY runner stderr deadlock and improve error handling

[shbhmexe]

Proposed Changes

This PR fixes reliability issues in the SU2 Python runner (SU2_PY/SU2/run/interface.py):

1. Stderr deadlock prevention - Use subprocess.communicate() to drain stderr while streaming stdout, avoiding potential hangs when SU2 processes write heavily to stderr.

2. Improved environment variable handling - Added explicit error message when SU2_RUN environment variable is not set.

3. Path reporting fix - Fixed typo in error messages (abspath(".") instead of abspath(",")) to show correct working directory.

Reasoning

• The Python runner previously waited for process exit before reading stderr. If stderr fills its pipe buffer, the child process can block indefinitely.
• Missing SU2_RUN now gives a clear error message instead of a confusing KeyError.

Impact

• No behavior change for normal runs
• Prevents rare but severe runner hangs on large stderr output
• Better error messages for debugging

Verification

• Tested: python -m compileall -q SU2_PY ✅
• No changes to solver algorithms or config parsing

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings.
• My contribution is commented and consistent with SU2 style.
• I used the pre-commit hook to prevent dirty commits.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation, if necessary.

[shbhmexe]

Hey @pcarruscag,
Would appreciate if you could take a look at this PR when you have time. #2650 , #2636
Thanks!

[pcarruscag]

Update the title and description please

[shbhmexe]

Update the title and description please

Hi @pcarruscag ,

Updated the title and description. Thanks for the review!