Investigate FMI extension dependency specialization (Issue #3905)

This commit documents an investigation into replacing FMI.jl with
FMIImport.jl as the dependency for the MTK FMI extension.

## Investigation Summary

Issue #3905 suggested that the FMI extension could be specialized to
depend on FMIImport.jl instead of the broader FMI.jl package, since
the extension "only needs import functionality".

## Findings

The proposed change is **not feasible** due to significant API
incompatibilities between FMI.jl and FMIImport.jl:

### API Differences

1. **Generic vs Version-Specific Functions**:
   - FMI.jl provides generic functions like `getValueReferencesAndNames()`
   - FMIImport.jl only provides version-specific functions like
     `fmi2GetValueReferencesAndNames()` and `fmi3GetValueReferencesAndNames()`

2. **Missing High-Level Wrappers**:
   - The MTK extension relies on several high-level convenience functions
     that exist in FMI.jl but not in FMIImport.jl
   - FMIImport.jl is a lower-level package focused on core import functionality

### Attempted Implementation

An attempt was made to replace FMI.jl with FMIImport.jl by:
- Updating Project.toml dependencies
- Using `import FMIImport as FMI` for API compatibility
- Updating test dependencies

This approach failed because:
- The generic `getValueReferencesAndNames` function doesn't exist in FMIImport
- The extension tries to define missing methods, causing compilation errors
- Many other high-level functions would also need to be reimplemented

## Conclusion

FMI.jl and FMIImport.jl serve different purposes in the FMI ecosystem:
- **FMI.jl**: High-level interface with convenient wrapper functions
- **FMIImport.jl**: Lower-level core import functionality

The MTK extension requires the high-level interface provided by FMI.jl.
Changing to FMIImport.jl would require substantial refactoring of the
extension to use only low-level APIs, which is beyond the scope of the
original issue.

**Recommendation**: Keep the current FMI.jl dependency as it provides
the necessary high-level API that the extension relies on.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas