Fix #3421: Guard DT_mooring validation with mooring enabled check

[andrew-platt]

Ready to merge

Feature or improvement description

This PR fixes a bug in FAST.Farm where DT_mooring validation was incorrectly executed regardless of whether shared mooring was enabled, causing fatal errors for users running cases with Mod_SharedMooring = 0.

The fix adds conditional guards around mooring-related validations so they only execute when mooring is actually enabled (Mod_SharedMooring = 3). Additionally, this implements the TODO for verifying that the MoorDyn input file exists.

Changes made:

• Wrapped DT_mooring validation in if (p%MooringMod == 3) conditional
• Added file existence check for MD_FileName when mooring is enabled
• Added file_exists logical variable declaration to support the validation

Before:

! --- SHARED MOORING SYSTEM ---
! TODO : Verify that p%MD_FileName file exists
if ((p%DT_mooring <= 0.0_ReKi) .or. (p%DT_mooring > p%DT_high)) CALL SetErrStat(ErrID_Fatal,'DT_mooring must be greater than zero and no greater than dt_high.',ErrStat,ErrMsg,RoutineName)

After:

! --- SHARED MOORING SYSTEM ---
if (p%MooringMod == 3) then
   inquire(file=trim(p%MD_FileName), exist=file_exists)
   if (.not. file_exists) call SetErrStat(ErrID_Fatal,'Cannot find MoorDyn input file '//trim(p%MD_FileName),ErrStat,ErrMsg,RoutineName)
   if ((p%DT_mooring <= 0.0_ReKi) .or. (p%DT_mooring > p%DT_high)) CALL SetErrStat(ErrID_Fatal,'DT_mooring must be greater than zero and no greater than dt_high.',ErrStat,ErrMsg,RoutineName)
end if

Related issue, if one exists

Fixes #3421

Impacted areas of the software

• FAST.Farm input file validation (glue-codes/fast-farm/src/FAST_Farm_IO.f90)
• Specifically the Farm_ValidateInput() subroutine

Additional supporting information

The implementation follows established OpenFAST patterns:

• Module-specific conditional validation (similar to CompMooring checks in FAST_Subs.f90)
• File existence checks (similar to pattern in WaveTank_IO.f90)

This is a non-breaking change that:

• ✅ Allows cases with Mod_SharedMooring = 0 to run without spurious DT_mooring validation errors
• ✅ Maintains strict validation when mooring is enabled (Mod_SharedMooring = 3)
• ✅ Adds better error reporting for missing MoorDyn input files

Generative AI usage

Co-authored-by: GitHub Copilot noreply@github.com
Co-authored-by: Claude Sonnet 4.5 noreply@anthropic.com

Test results, if applicable

• r-test branch merging required

No r-test changes needed. This is a validation-only fix that:

1. Removes false-positive errors when mooring is disabled
2. Adds file existence validation when mooring is enabled

Existing regression tests with mooring enabled should continue to pass. Cases with Mod_SharedMooring = 0 will no longer fail validation unnecessarily.

[rthedin]

Thanks for fixing this!