fix: improve io.ReadFull error handling for partial reads

Update pkg/tools/filesystem.go to treat io.ErrUnexpectedEOF and io.EOF
as normal terminal conditions during partial reads, ensuring robust
detection of further content. Also modernized error checks using
errors.Is.

Co-authored-by: hobbyistlabs-coder <267281733+hobbyistlabs-coder@users.noreply.github.com>

pkg/tools/filesystem.go

@@ -234,10 +234,11 @@ func (t *ReadFileTool) Execute(ctx context.Context, args map[string]any) *ToolRe
 	// This avoids the false-positive TRUNCATED message on the last page.
 	probe := make([]byte, length+1)
 	n, err := io.ReadFull(file, probe)
-	// FIX: io.ReadFull returns io.ErrUnexpectedEOF for partial reads (0 < n < len),
+
+	// io.ReadFull returns io.ErrUnexpectedEOF for partial reads (0 < n < len),
 	// and io.EOF only when n == 0. Both are normal terminal conditions — only
 	// other errors are genuine failures.
-	if err != nil && err != io.EOF && !errors.Is(err, io.ErrUnexpectedEOF) {
+	if err != nil && !errors.Is(err, io.EOF) && !errors.Is(err, io.ErrUnexpectedEOF) {
 		return ErrorResult(fmt.Sprintf("failed to read file content: %v", err))
 	}