fix(): Added File name validaiton for Surface integral

[benflexcompute]

Note

Strengthens validation for output naming to prevent invalid filenames.

• Introduces FileNameString with pydantic AfterValidator to reject /, null bytes, empty strings, and reserved names (./..)
• Updates SurfaceIntegralOutput.name to use FileNameString with a clarified description
• Adds tests (tests/simulation/outputs/test_filename_string.py) covering valid/invalid cases and SurfaceIntegralOutput name behavior

^{Written by Cursor Bugbot for commit 92a9e32. This will update automatically on new commits. Configure here.}