Fix is_file() to distinguish directories from files

[Copilot]

Configuration loading failed when /configs/dmclk was a directory containing INI files. The system attempted to parse the directory itself as an INI file, producing errors.

Root Cause

is_file() used Dmod_Access(path, DMOD_F_OK) which only checks existence, returning true for both files and directories.

Changes

• Modified is_file() to attempt opening the path as a directory
• Returns false if Dmod_OpenDir() succeeds (it's a directory)
• Returns true otherwise (it's a file)

static bool is_file(const char* path)
{
    if (Dmod_Access(path, DMOD_F_OK) != 0)
        return false;
    
    void* dir_handle = Dmod_OpenDir(path);
    if (dir_handle != NULL)
    {
        Dmod_CloseDir(dir_handle);
        return false;  // It's a directory
    }
    
    return true;  // It's a file
}

This enables configure_drivers() to correctly recurse into configuration subdirectories and process INI files within them.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>dmclk nie może być skonfigurowany przez niepoprawne szukanie plików konfiguracyjnych</issue_title>
<issue_description>Oto logi:

[INFO] Connecting API from dmell to dmdevfs
[INFO] Connecting API from dmramfs to dmdevfs
[INFO] Connecting API from dmffs to dmdevfs
[INFO] Connecting API from dmini to dmdevfs
[INFO] Connecting API from dmdevfs to dmell
[INFO] Connecting API from dmdevfs to dmramfs
[INFO] Connecting API from dmdevfs to dmffs
[INFO] Connecting API from dmdevfs to dmdevfs
[INFO] Connecting API from dmdevfs to dmini
[INFO] Module enabled: dmdevfs
[INFO] dmlist: Created list for module dmdevfs.
[INFO] Directory '/configs' opened successfully
[ERROR] Failed to open file '/configs/dmclk'
dmdevfs: [ERROR] Failed to parse INI file: /configs/dmclk
dmdevfs: [ERROR] Failed to read driver for config: /configs/dmclk
[INFO] Directory closed successfully
[INFO] File system 'dmdevfs' mounted at '/dev' successfully
[INFO] No startup.dmp embedded in ROM
DMOD-Boot started
stm32f746xg@/> ls /configs/dmclk
[WARN] Could not create mutex
stm32f746g-disco.ini  
[WARN] dmheap: Module Invalid is not registered.

```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes dmclk nie może być skonfigurowany przez niepoprawne szukanie plików konfiguracyjnych #16

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