fix(stdlib_system): use STARTUPINFO for windows stdin redirection

[JAi-SATHVIK]

Problem:-

The previous implementation attempted to redirect standard input by appending the shell operator < filename directly to the command string (e.g., cmd < input.txt).

The Problem: The Windows API function CreateProcess does not spawn a shell (like cmd.exe). Consequently, it does not recognize or interpret shell operators.

The Result: The < character and the filename were passed as literal arguments to the child program. This caused the child program to receive "garbage" arguments while the actual stdin stream remained unassigned.

Solution:-

Removed Incorrect Syntax: The code no longer appends < filename to the command line string.

Native Handle Management: Utilizes CreateFile to open a legitimate, read-only handle to the specified input file (or NUL if no file is provided).

Process Inheritance: The handle is assigned to the hStdInput field of the STARTUPINFO structure.

The STARTF_USESTDHANDLES flag is enabled to instruct the OS to use the provided handles.

Resource Cleanup: Proper logic was added to ensure the file handle is closed safely after the process is launched, preventing resource leaks.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 68.69%. Comparing base (af58730) to head (d2fdd50).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
test/system/test_subprocess.f90	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1079      +/-   ##
==========================================
+ Coverage   68.60%   68.69%   +0.08%     
==========================================
  Files         393      393              
  Lines       12710    12720      +10     
  Branches     1377     1376       -1     
==========================================
+ Hits         8720     8738      +18     
+ Misses       3990     3982       -8     

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[jalvesz]

Thanks for catching this @JAi-SATHVIK!

The fact that you catched such a flagrant bug when the CI didn't brings me to the conclusion that the tests in place for this functionality are not thorough enough. Would it be possible to add a unit test that would ensure the correct behavior of the API?

[perazz]

Thank you for the catch! LGTM overall. Before merging, I suggest:

• Make stdin redirection work in all cases: on Windows the child can only use hStdInput if the underlying handle is allowed to be “passed” to the child process. Please open both the real input file and the fallback NUL handle in a way that allows passing to the child, and ensure CreateProcess is called with handle passing enabled.
• Test: add a small Windows test that redirects stdin from a file and verifies the child reads expected content.
• Cleanup/leak: close any opened handles on all early-return/error paths to avoid leaks.

[jvdp1]

With 2 approvals, I will merge this PR.
Thank you @JAi-SATHVIK for fixing this bug.
Thank you @perazz and @jalvesz for your reviews.