Attach Paradox blob file on open and enable pxfBLOb writes

[ivofcups]

Summary

This PR improves Paradox blob handling in PxSession.cpp by attaching the external blob file during session open and expanding blob write support.

Changes

• Call PX_set_blob_file() after PX_open_file() succeeds

• Reset session state before opening:

  • set open = false
  • clear blobfilepath

• Resolve blob file using either .mb or .MB

• Set open = true only after initialization succeeds

• Extend SetRowCol() to handle pxfBLOb in addition to pxfFmtMemoBLOb and pxfMemoBLOb

Why

Previously, the session opened the .DB file but did not attach the associated blob file, which could prevent correct handling of blob-backed fields. This change ensures the blob file is associated when present and broadens write support for generic blob fields.

Testing

• Built the project on Windows with MSYS2/MinGW

• Verified the changes compile successfully

• Confirmed the diff includes:

  • blob file attachment in Open()
  • pxfBLOb handling in SetRowCol()

Risks / Notes

• Blob attachment is only attempted when a matching .mb or .MB file exists
• If blob attachment fails, Open() returns false
• This change is limited to session initialization and blob-field write routing