How do I script the Paint Bucket?

If Trim using the top left or bottom right corner do not work adding a white pixel there may not work either.  I do not know how trim works or how much tolerance it may use.  I would suggest trying to simulate a trim with some high tolerance using the magic wand. If its a background layer image. Convert it to a normal layer and  add a white layer under that layer the use the magic wand tool to select contiguous pixels sampling areas on the boarder of the canvas  invert and clear pixels on that layer.  Do this several time sample different point about the canvas border,   Then get the layer's  bounds and crop the canvas to that layers bounds..

I wrote a script something like that to trim layer that have borders there was no cropping done.

try { if(app.documents.length>0){ app.activeDocument.suspendHistory ('Trim Layers', 'TrimLayers();') } }
catch(e) { alert( e + ': on line '  + e.line );}
function TrimLayers() { 
  var orig_ruler_units = app.preferences.rulerUnits; // save user ruler units  
  var orig_display_dialogs = app.displayDialogs; // Save users Displat Doalogs
  app.preferences.rulerUnits = Units.PIXELS; // set the ruler units to PIXELS
  app.displayDialogs = DialogModes.NO; // Set Dialogs off
  var doc  = app.activeDocument; // get current document 
  var saveactiveLayer = doc.activeLayer; // save activeLayer
  doc.selection.deselect(); // make sure ther is no active selection
  try {processArtLayers(doc);} // Process all Layers
  catch(e) { alert( e + ': on line '  + e.line );} // some unexpected error
  doc.selection.deselect(); // make sure ther is no active selection
  try { doc.activeLayer = saveactiveLayer;} // restore activeLayer
  catch(e) { } // may have been deleted
  app.displayDialogs = orig_display_dialogs; // Reset display dialogs 
  app.preferences.rulerUnits = orig_ruler_units; // reset units to original settings
}
///////////////////////////////////////////////////////////////////////////////
// Function: processsArtLayers
// Input: document or layer set
///////////////////////////////////////////////////////////////////////////////
function processArtLayers(obj) {
    for(var i = obj.artLayers.length-1;0<= i; i--) { // for Layers
  try {TrimLayer(obj.artLayers); } // process Art Layer
  catch(e) {} // don't stop on errors
  }
    for(var i=obj.layerSets.length-1;0<=i;i--) { // for Layer sets
  try {processArtLayers(obj.layerSets);} // process Layer Set
  catch(e) {} // don't stop on errors
  }
}
function TrimLayer(layer) { 
  // do nothing if layer is background or locked  
  if(layer.isBackgroundLayer || layer.allLocked || layer.pixelsLocked || layer.positionLocked || layer.transparentPixelsLocked ) return; 
  //if(layer.protectArtboardAutonest ) return;
  if( layer.kind != LayerKind.NORMAL) return; // do nothing if not normal layer   
  app.activeDocument.activeLayer = layer; // target layer
  var bounds = layer.bounds; // layer bounds  
  var LWidth = bounds[2]-bounds[0]; // layer width  
  var LHeight = bounds[3]-bounds[1]; // layer height 
  var Lleft =  bounds[0]; // layer left x
  var Lright =  bounds[2]; // layer right x
  var Ltop =  bounds[1]; // layer top y
  var Lbottom =  bounds[3]; // layer bottom y
  var tlrnc = 20; // tolerance
  i=0.6; // fudge factor
  try {
  MagicWand((Lleft+i), (Ltop+i), tlrnc); // top left
  app.activeDocument.selection.clear(); // clear
  }
  catch(e) {}; // do nothing
  try {
  MagicWand((Lleft+LWidth/2), (Ltop+i), tlrnc); // top middle
  app.activeDocument.selection.clear(); // clear
  }
  catch(e) {};   // do nothing
  try {
  MagicWand((Lright-i), (Ltop+i), tlrnc); // top right
  app.activeDocument.selection.clear(); // clear
  }
  catch(e) {}; // do nothing 
  try {
  MagicWand((Lleft+i), (Ltop+LHeight/2), tlrnc); // middle left
  app.activeDocument.selection.clear(); // clear
  }
  catch(e) {}; // do nothing 
  try {
  MagicWand((Lright-i), (Ltop+LHeight/2), tlrnc); // middle right
  app.activeDocument.selection.clear(); // clear
  }
  catch(e) {}; // do nothing 
  try {
  MagicWand((Lleft+i), (Lbottom-i), tlrnc); // bottom left
  app.activeDocument.selection.clear(); // clear
  }
  catch(e) {};   // do nothing
  try {
  MagicWand((Lleft+LWidth/2), (Lbottom-i), tlrnc); // bottom middle
  app.activeDocument.selection.clear(); // clear
  }
  catch(e) {}; // do nothing 
  try {
  MagicWand((Lright-i), (Lbottom-i), tlrnc); // bottom right
  app.activeDocument.selection.clear(); // clear
  }
  catch(e) {};   // do nothing
}
function MagicWand(x,y,tolerance) {
var idsetd = charIDToTypeID( "setd" );
    var desc106 = new ActionDescriptor();
    var idnull = charIDToTypeID( "null" );
        var ref60 = new ActionReference();
        var idChnl = charIDToTypeID( "Chnl" );
        var idfsel = charIDToTypeID( "fsel" );
        ref60.putProperty( idChnl, idfsel );
    desc106.putReference( idnull, ref60 );
    var idT = charIDToTypeID( "T   " );
        var desc107 = new ActionDescriptor();
        var idHrzn = charIDToTypeID( "Hrzn" );
        var idPxl = charIDToTypeID( "#Pxl" );
        desc107.putUnitDouble( idHrzn, idPxl, x );
        var idVrtc = charIDToTypeID( "Vrtc" );
        var idPxl = charIDToTypeID( "#Pxl" );
        desc107.putUnitDouble( idVrtc, idPxl, y );
    var idPnt = charIDToTypeID( "Pnt " );
    desc106.putObject( idT, idPnt, desc107 );
    var idTlrn = charIDToTypeID( "Tlrn" );
    desc106.putInteger( idTlrn, tolerance );
    var idAntA = charIDToTypeID( "AntA" );
    desc106.putBoolean( idAntA, true );
executeAction( idsetd, desc106, DialogModes.NO );
}