fix validation of long strings

[jcupitt]

We were failing silently if string length validation failed. With this PR, issue a warning and let long strings though.

Also, add a -w flag (show warnings) to dcm-dump and dcm-getframe.

Thanks @arngaillard

See #105

[jcupitt]

With this PR, libdicom warns (it was failing silently) for string length validation failure and passes the out of spec string downstream.

I now see:

$ vipsheader converted.0_0_0.dcm
converted.0_0_0.dcm: 51200x38144 uchar, 4 bands, srgb, openslideload

ie. everything works by default.

I added a -w flag to display warnings and with that I see:

$ dcm-dump -w converted.0_0_0.dcm > log
WARNING  [Tue Oct 21 15:35:04 2025] - Data Element capacity check failed -- Value of Data Element '00280030' exceeds maximum length of Value Representation (16)

And the dcm-dump output shows:

(5200,5200) SharedFunctionalGroupsSequence [
  ---Item #1---
  (0028,0028) PixelMeasuresSequence [
    ---Item #1---
    (0018,0050) SliceThickness | DS | 2 | 1 | 1
    (0018,0088) SpacingBetweenSlices | DS | 2 | 1 | 1
    (0028,0030) PixelSpacing | DS | 42 | 2 | [4.564125969876768E-4, 4.550625711035267E-4]
  ]
  (0040,0040) WholeSlideMicroscopyImageFrameTypeSequence [
    ---Item #1---
    (0008,9007) FrameType | CS | 28 | 4 | [DERIVED, PRIMARY, VOLUME, NONE]
  ]
]

ie. the out of spec value is handled correctly.

[jcupitt]

@arngaillard has tested this and it seems to work, I'll merge.