Found and fixed a type cast missing in FAST_writer.py

[andrew-platt]

Ready to merge

Feature or improvement description

This PR fixes a TypeError in FAST_writer.py when writing ElastoDyn blade files with a blade index. The issue occurred at line 620 where string concatenation with an integer was performed without proper type casting.

Changes:

• Added str() cast around (bldInd+1) to properly convert integer to string before concatenation
• Fixes the crash in writer.execute() when write_ElastoDynBlade() is called with a blade index parameter

Related issue, if one exists

Fixes #3431 -- reported and fixed by @jcvem

Impacted areas of the software

• openfast_io/FAST_writer.py - ElastoDyn blade file writer
• Specifically affects the write_ElastoDynBlade() method when writing blade files for indices 1 and 2

Additional supporting information

The bug was reported by @jcvem and caused a TypeError when trying to write ElastoDyn blade input files. The issue was that when bldInd is not None, the code attempted to concatenate a string 'BldFile' with an integer (bldInd+1) without converting it to a string first.

Before:

blade_file = os.path.join(self.FAST_runDirectory,self.fst_vt['ElastoDyn']['BldFile'+(bldInd+1)])

After:

blade_file = os.path.join(self.FAST_runDirectory,self.fst_vt['ElastoDyn']['BldFile'+str(bldInd+1)])

Generative AI usage
None

Test results, if applicable

This is a simple type cast fix that does not affect the logic or output of the code, only prevents a runtime error. No regression test updates are expected to be necessary.